Peer pressure is a terrible thing. It is nine o'clock on a Saturday morning and, rather than being in bed, which is where I'd like to be, I'm standing in the lobby of an anonymous hotel in West London. Why? Because my friend, N, has persuaded me - who knows how - to attend a Paul McKenna Weight Loss Event. 'Come on!' she said. 'It's right up your street. You can lose weight without doing anything at all!' In fact, this is not entirely accurate, but her appeal was clever, and well-aimed. Just like Paul McKenna, I hate diets, and I've never been on one. I also love eating. For a female, I'm as unscrewed-up about food as it's possible to be. There is nothing I avoid (apart from celery, which is vile), and no 'low-fat' product that I would be tempted to buy. However, if I am absolutely honest, like every woman I've ever met, I sometimes do dumb things. Occasionally, especially when I'm working, I eat nothing all day and then, just as I'm about to faint, I'll scoff eight biscuits. I eat when I'm bored, and when I'm miserable. And guess what? I wouldn't mind losing a stone. Show me a woman who doesn't want to lose a stone, even if only in theory, and I'll show you ... well, a bloke.

So, here we are. Me, N, and about 500 people - women, mainly - who want to lose weight. It costs £250 to attend this event, plus travel, so it's a serious investment for most people - and that's exactly how they see it, as an investment. This crowd is not, you quickly realise, here to be entertained. For the majority, this is the last resort. They have nothing to lose. They have tried everything, from calorie-counting to drinking cabbage soup, and they are still fat. Who can blame them for thinking that Paul McKenna, hypnotist, self-proclaimed 'expert on the power of the human mind' and author of the best-selling I Can Make You Thin, might be worth a punt? Not only does his book have the most alluringly explicit title ever published in the field of self-help, but he claims that his 'system' also has a success rate of 'over 70 per cent' (Kirsty Young, the newsreader, is one of his successes. She even gave him a blurb for his book. 'I lost weight long-term and re-established a relaxed relationship with food.') This live version of the book, then, has double its appeal: you get all the benefits of the 'system' - immediate and life-long weight loss, plus increased self-esteem - but without even having to read about how to achieve them. Plus, you get Paul to hold your hand! To shout his encouragement! To tell you you're lovely! You can see why this might appeal to the lonely dieter, worn out with circling the biscuit tin in her kitchen.

And boy, is he encouraging. We register, and go through into a big function room. When McKenna first appears on his specially assembled stage - in black suit and glasses, and wearing a smile that says: 'I've brushed, I've flossed and I'm ready to save your life' - the atmosphere in the room is lacklustre - depressed, even. People are sitting around, feeling rubbish about their thighs, and waiting for the miracle to happen. No matter. He has energy enough for everyone. It's Mr Bean meets Batman up there! McKenna's 'system', as he would be the first to admit, is very, very simple. It consists of four golden rules. Follow them, and you will lose weight. One: when you are hungry, eat. Two: eat what you want, not what you think you should. Three: eat consciously, and enjoy every mouthful. Four: when you think you are full, stop eating. That's it.

But, of course, following these rules is easier said than done, and this is where McKenna's beloved neuro-linguistic programming comes in. He has all sorts of weird ways of getting you to change your behaviour, most of which have to do with what he calls the 'reprogramming of the mind'. Here's an example. A volunteer comes up on stage, a chocolate addict. McKenna produces a giant bar of Dairy Milk and watches her salivate. Then, over a period of minutes, he builds up an association in her mind between chocolate and a food she really hates - a food, what's more, that has also been covered with hair from the floor of a barber's shop. Ugh! The latter makes her gag and, once the association is fully established, so does the chocolate. 'How about a piece now?' he says, snapping off a corner. She pulls a face. The audience gasps.

Another much weirder technique favoured by McKenna is called 'tapping'. In fact, he has a special film to show us about it, in which a cola addict uses it to beat her cravings. Tapping was developed by Dr Roger Callahan, author of Tapping the Healer Within, and it involves tapping on certain acupuncture points in the body. Think about whatever it is that you crave. Now use two fingers to tap above your eyes 10 times, then below them, under the collarbone, in the armpit, and on the back of your hand. Close your eyes, open them, look down to the right, down to the left, then rotate them 360 degrees in either direction. Count out loud from one to five, and hum the first few lines of 'Happy Birthday'. Now tap under your eye again, and under your collarbone and armpit... OK, how is the craving? According to McKenna, it will either be vastly reduced or gone altogether (he does not have any advice as to what you should do if you happen to be in a public place when this routine is called for). If not, go through the sequence again. McKenna loves tapping. So do his trained volunteers, who stand at the back waiting to help us out when we work in groups. For some of them, it seems to be a kind of all-purpose cure. When I try to leave early because I have a toothache, one of them catches me and, after I explain the problem, suggests that I 'tap the toothache out'. Oh, come on. Just show me the paracetamol.

It's a long and exhausting day. Repetition is always tiring and part of McKenna's technique, honed during his heyday as a stage hypnotist, is to repeat his message over and over in as many different forms as possible. There is no real way of judging the impact of his efforts on his audience; no follow-up work is done afterwards, so who really knows if this event will change forever their dysfunctional relationship with food? But I will say that, by the afternoon, people look happier and more bright-eyed. Me? I'm impressed by his shtick, and the inherent common sense of his message, but I'm still thinking - obsessing would be a better word - about the piece of beef that I plan to roast when I get home. I'm also worried about N. Just now, I looked at her out of the corner of my eye, and her mouth was wide open in wonderment. During our lunch break (food is not included in the cost of the event), she keeps telling me to put my knife and fork down (McKenna wants us to eat slowly, the better to enable us to hear the signal from our brain to our belly telling us that we are full), and to chew everything 12 times. 'Prawns disappear after three chews,' I say. 'No they don't!' she replies. 'Just do as you are told! Don't you want to lose weight?'

The following week, having decided that I want to write about McKenna, I have a private consultation with him at his Kensington mews house. It's very amusing. What a smoothy! He greets me first in his office, which features a life-size Paul McKenna cardboard cut-out, lots of Paul McKenna show posters, a box containing a model of a UFO and a Great Dane with unfeasibly huge testicles. He then takes me upstairs to his, er, private quarters, which are sort of 1980s minimalist: there are Oriental touches, and lots of leather furniture. In his study, where we sit for my consultation, there are button-back armchairs, stacks of self-help books by other people and, on the back of a cupboard door, a full-length mirror.

McKenna is incredibly fidgety, jiggling his knee and rubbing his nose, and it's not very relaxing. He asks me about my eating habits. I tell him that I was brought up to clean my plate, and that it makes me anxious to leave it otherwise. We work on this anxiety. He tells me to imagine myself in a liberating movie in which the sight of an unfinished roast potato means absolutely nothing to me. We also practise our tapping. Then he gets me to stand in front of the mirror. I'm not happy about this. The lighting is highly unflattering.

'So, Rachel,' he purrs, in his weird pop-picker twang (McKenna is a former DJ). 'What do you see? What do you dislike about yourself?' I rattle through the usual: face, arms, legs, bum, er, belly. I concede that my breasts aren't bad. McKenna takes a deep breath and does a routine I saw him do at the event. He adopts a frankly ridiculous voice, like a dwarf with adenoids trying for a role on 'Allo 'Allo. 'Ah hate ma bum!' he says, mincing and gurning. 'Ah hate ma face! Ah hate ma arms! Ah hate ma legs!' I don't know where to put myself. The idea is that I will see how silly it is to be so self-critical, but he's the one who seems daft, not me. It's embarrassing. Will I hear this voice every time I scrutinise my cellulite? I hope not. He then asks me to think of a compliment someone once paid to me, to remember how it made me feel, and look at myself again. 'See! You're already standing differently,' he says. This is true, but I'm not sure it has anything to do with my brain. I just want to please Paul. Sure, he's cheesy, but he's also so determined, so convinced by his self-appointed mission. I can't bear to let him down. I'm very approval-seeking, that way.

He sends me off with a full collection of his weight-loss CDs and a 'success journal', in which I can record my habits. I'm disappointed that he didn't hypnotise me, and I'm not convinced that our little chat is going to have any effect. Then something weird happens. I don't start thinking I am Christy Turlington but, over the next few days, I notice that I eat more slowly, and feel full more quickly. This involves no effort on my part; it just happens.

By the end of the following week, my trousers fit better. I'm pleased by this, but also confused. I think of myself as a rational person. I do not believe in 'mind reprogramming', and even if I did, I don't think I'm the kind of person on whom it would work; I'm cynical and stubborn and not easily led. What is going on? Am I secretly following the rules, but refusing to admit that I am? No. The full feeling comes upon me from nowhere, and I'm certainly not doing any tapping (though when Paul rings me to find out how I'm doing, I lie and say that I am). This leaves me with only two alternatives: either he hypnotised me on the sly, and I just didn't notice or, God forbid, he really has 'reprogrammed' my mind.

A month later, I visit McKenna again. I'm still not eating as much as I usually do, though some of my weight loss is undoubtedly down to the fact that I've been in Iran, and thus very much off the booze. We sit on his roof terrace, in the sunshine, observed by the couple opposite on their roof terrace, who seem to regard our encounter as a live show. I tell him I've lost weight, and he nods, Bean-ishly - well, of course I have! So what did he do to me? 'It's more what you have done to yourself with my assistance,' he says. 'You've broken the habits of the past. The signal [from brain to belly] is coming on strong.' But if his techniques are really so fast and effective, why don't more people use them? Why does anyone bother with, say, cognitive behavioural therapy, which takes months to deal with anxieties and phobias? He sighs. McKenna is almost as down on the world of therapy as he is on the world of dieting. 'It takes forever. Ye olde traditional counsellors. These people are well-intentioned, but it seems to me that getting people to go back to trauma isn't a comfortable experience.' So is he a believer in repression? 'Absolutely not. That implies that you've put a lid on something, and it's there, bubbling away. This is about recoding.'

What I want to know is: why didn't he just hypnotise me (assuming, of course, that he didn't)? He is reputed to have helped numerous celebrities - Robbie Williams, Sophie Dahl and Geri Halliwell among them - with problems as diverse as writer's block to giving up smoking using hypnosis. 'Well, my book is written in hypnotic language. But modern hypnosis is different to the stuff you know from the stage. It's more conversational. It's a feedback loop. I'm taking where you're at emotionally to determine the pace and style of the change. Then I set up patterns of agreement. Things you can say "yes" to right at the beginning, hypnotic metaphors that you can relate to. But this is a system; the system isn't just hypnosis. I've had good results for weight loss using hypnosis, but I find that there are certain conscious behaviours people can improve as well. Give a man a fish, and he eats for a day; teach a man to fish, and he eats for life. To some extent I want people to put on the CD [the mind-programming CD that comes with his book] and just change. Woosh! But it's also good if people get their sense of control back. I am on a quest to close down the hate-your-body industry. That's why it's better to spend time with 500 people than doing one-on-ones. It's more profitable, and I don't just mean commercially.'

If this all sounds a bit vague, perhaps that's because it is. McKenna is even more fidgety than last time I saw him, and tired from a long day of meetings. He seems to have forgotten the details of our last encounter. Never mind. There's something unexpectedly likeable about him, in spite of his craaaa-zy verbal ticks and his tendency to Americanise his voice when he's in explanatory mode. For one who made his name in such a rum business - stage hypnosis has a questionable reputation, which is one reason, I suspect, why he has left it behind him, along with the waistcoats - he is intensely straightforward. Does he like being famous? 'Yes, I really like it.' Is he rich? 'Yes.' Would he like to be the biggest self-help guru in the world? 'Yes.' It is almost impossible to embarrass him. He takes pretty much everything as a compliment. Your patter is so slick, I say. 'Thank you,' he replies. Where does this confidence come from? Well, to give himself a boost, he does something called Big Mind, which was developed by a Western Zen master, Dennis Merzel. 'It shortcuts straight into the nirvana process. You forget about yourself and your consciousness expands and becomes infinite.' The voice drops; suddenly, he is doing a movie trailer. 'It's a feeling of euphoria, bliss and immense inner peace. Problems dissolve. I do it every day.'

McKenna is from Enfield, in suburban north London. His father was a builder, his mother a cookery teacher. Although he left school at 17, with few qualifications, he always knew he was going to be famous. 'I knew it from the age of seven. I thought I'd be a rock star, or something to do with communication. It's weird, isn't it?' He worked first as a DJ in Topshop, and then at Radio Caroline, Chiltern, Capital and, finally, Radio 1. But by this point, he was already a hypnotist by night (he fell for its power after he interviewed a hypnotist while he was at Radio Chiltern). He decided to give up DJing, and do the hypnotism full-time. 'I asked myself where I would end up if I continued on the same course. I'd be older, balder, more paranoid, radio would change. Then I asked myself what I'd do if I knew that I couldn't fail. I'd do hypnotism, have a show on TV and glamorous celebrity clients and travel the world. So I went and fucking quit.' Was he a good DJ? 'Yes. I would have got the breakfast show if I'd stayed.' Within a few months, he'd piloted a TV show, The Hypnotic World of Paul McKenna, which began in 1993 and regularly attracted 12 million viewers.

But then it was all change again. McKenna denies that, if he'd stayed in showbusiness, he would have ended up 'like Paul Daniels' - but still, he decided to build up the more serious side of his empire. Within a couple of years, Paul McKenna Training was the biggest hypnosis and neuro-linguistic training centre in the world (neuro-linguistic programming was first developed by Richard Bandler and John Grinder in the 1970s; in 1994, McKenna approached Bandler and asked if they might go into business together, which they did; at the heart of Bandler's philosophy is 'modelling' - if someone has a skill that you want to master, you 'model' it, so you can learn to do what they do, only in a fraction of the time it took them). Now, having written a series of bestsellers based on these techniques, McKenna is preparing for the next stage of his plan for global domination. He is currently working on ideas with a hot-shot American TV producer. 'I've got a massive vision. Psychological make-over TV is the next big area. That's where I'm headed. We'll show people changing on screen. But things will also happen [to viewers] through the barrel on the lens, and with web back-up.' Crikey. So he's literally going to invade people's homes? 'Absolutely.'

McKenna is a content serial monogamist, who favours leggy blondes (past girlfriends include the TV presenter Penny Smith and the model Liz Fuller, who dumped him live on her cable-TV show). His current partner is 'a dog person who's training to be an animal behaviourist'. Do his girlfriends expect them to sort out their problems? 'Ooh, yeah, but I pick some like that on purpose. It makes me feel better about myself because I can go: I can help you with this.' Has he ever hypnotised a woman in order to make her go out with him? 'I wish.'

A friend of his does something called speed seduction, which uses hypnotic language to hold their attention. 'He asks them if they've ever been in love, and what they felt like, and then attaches himself to that feeling. Women find it very offensive because it's very chauvinistic.' Hmm. So when he tells the overweight women who come on stage at his events that they are beautiful, does he believe it? What does he really see? 'I just see layers of sadness, and I want to take it all away. I see the essence of who they are, and the story that they tell themselves, not like a psychic or anything, but because I've been doing this for a long time.' At this point, he starts to quote Mother Teresa, but I am going to save him from himself, and resist the temptation to repeat these thoughts in print.

It almost time for me to go now; he's on QVC tonight, selling his weight-loss system, so he probably needs a bit of a breather. But he tells me to keep listening to my CDs, especially Overcome Emotional Eating and to keep tapping, and to just wait and see: he is fully expecting that I will shed more pounds in the coming weeks. He promises to call me, very soon.

In the end, Paul doesn't call. Nor, three weeks after our meeting, have I lost any more weight. But I am still more restrained in my habits, and I often hear his voice telling me to slow down, to really enjoy what I'm eating. The other day, I threw a piece of cold chicken away. I'm from Yorkshire! This is unprecedented! I can't explain how McKenna's system works, or even prove that it does (his statistics, so far as I can tell, are unreliable in the sense that there is no control group as a point of comparison and there are too many variables at play - in my case, hot weather and a trip to Iran - ever to isolate his influence as being of prime importance). But my hunch is that for some people it can and does work, that his 'hypnotic techniques' somehow allow common sense to prevail.

One of the success stories at the event I attended - a girl who came up on stage waving a pair of trousers so giant she could now fit into them three times - described this as 'things suddenly clicking into place'. Perhaps some people, when it comes down to it, just need a good talking to. Do I worry that he charges people £250 to hear him state the bleeding obvious? A little. But he's right about one thing. Diets do suck. If he can dent that wretched industry, he'll have done us all a favour.

· I Can Make You Thin by Paul McKenna (Bantam) is available for £9.99 from observer.co.uk/bookshop. Paul McKenna's weight-loss seminars are on 11 July, 15 September and 24 November. www.paulmckenna.com, 0845 230 2022

The rules according to Paul McKenna

1. Eat when you are hungry

'Starving yourself can actually make you fat. Not eating slows down your metabolism, which makes you feel tired. It can also lead to false hunger signals and subsequently, binge-eating. You need to train yourself to eat only when you're hungry.'

2. Eat what you want - not what you think you should

'As soon as you tell yourself to avoid certain foods, you upset the balance of your relationship with them. By eating what you want, you establish a balanced diet. Your tastes change and you may find yourself craving the very foods you're "supposed" to be eating. If you want it, eat it. Resistance is futile.'

3. Eat consciously - enjoy every mouthful

'People who are overweight often eat too quickly in order to get a serotonin high. Eating "subconsciously" can expand your stomach and cause weight gain.

Eat what you want, whenever you want, so long as you enjoy every bite. Chew every mouthful, slowing your eating speed down to a quarter of what it used to be, and you'll automatically eat less and feel better.'

4. When you think you are full, stop eating

'When you've eaten enough, your body should receive a signal in your solar plexus, which says it's satisfied. The more you pay attention to this, the more satisfied you will feel and you'll know when to stop eating. You need to re-sensitise yourself to your 'inner thermostat' so you stop eating food when you're full.'