Once, the phrase "good posture" brought to mind either a violently perpendicular, ramrod-backed sergeant major stance, or a deportment class full of debutantes balancing books on their heads.

Nowadays, the definition of good posture cleaves to a far more natural ideal. "Ideally, your body should be aligned so that if you were to drop an imaginary plumb line from your ear it would pass down your neck, through your shoulder, hip and knee and just in front of your ankle into your heel," says Martin Knight, a consultant spinal surgeon and director of the Spinal Foundation charity. "There should be a gentle curve to the lumbar [lower] spine, known as lordosis."

But if maintaining good posture seems simple in the abstract, the reality is very different. Modern life is filled with activities that could have been designed with the express purpose of causing us musculo-skeletal grief. My own waking hours are a case in point.

Like most people, I spend around 60% of them hunchbacked over a desk, occasionally by way of vertebral variation cradling the phone between my neck and shoulder while typing ever onwards. This is the kind of thoughtless behaviour that explains why two-thirds of the working population suffer some kind of back pain and that campaigns such as the recent Straighten Up UK, by the British Chiropractic Association, are valiantly trying to discourage. Like all of us, I poke my head towards the computer or TV screen for extended periods without acknowledging that the slender stalk of my neck was not designed to carry such a grotesquely heavy weight - the human head can weigh as much as 12lbs (5.4kg) - at such an angle for any length of time. And although my hamstrings have been saved much abuse by the fact that I am not a habitual high heel wearer, that other enemy of good female posture - the handbag that weighs even more than your head and gleefully pulls the spine off course with every step - is my constant companion.

What can be done? The first thing to note is that although our sitting and standing positions are important, they do not exist in isolation - which is to say that good posture is not simply a static thing. As fitness instructor Jayne Nicholls puts it, "I can teach you to stand with your pelvis in neutral and your stomach pulled in, but how much use is that? How many times a day are you standing still? Good posture is a dynamic matter as well, enabling you to perform efficiently and comfortably in all areas of your life." In other words, you are more likely to end up with less damaging ways of sitting, standing, working, driving etc if you have an integrated understanding of your body and learn to use it properly in the first place.

The most popular ways of doing this are currently yoga, Pilates and the Alexander technique. Many people head for classes on advice from their GP or other medical expert after a problem has developed. Others, like me, sign up as a pre-emptive strike against a modern lifestyle they realise offers them every chance of ending up looking like the bastard offspring of Quasimodo and the Witch of Endor, and spending their declining years shuffling crab-like towards the chemist for those repeat painkiller prescriptions unless they engage in some prophylactic exercise now. But how do they each measure up?

Yoga

"You have to be taught by someone who really knows what they are doing," says Knight. "If you already have a problem, they have to be able to identify exactly where that problem is. And it won't necessarily make a bad back better. There is a danger with [inexpertly supervised] yoga that you will simply stretch what will stretch and keep the rest stiff."

Overall, Knight thinks it inadvisable to say that a healthy person could and should do yoga, at least without a thorough MOT from a medical expert. "If you are not taught to move properly, the spine will take that punishment for a long time - because it is so brilliantly designed - but there will come problems, possibly years later. Don't be led into thinking that because there is no pain now, it cannot be damaging."

Pilates

Exercises that do not seek to improve flexibility offer less chance of injury to the unwary, and Pilates is frequently offered as part of post-operative rehabilitation to patients. "It can help people improve their posture, provided that their postural problems are purely muscular," says Tim Hutchful, of the British Chiropractic Association. "It's not so good for joint problems. And a lot of muscle pain can actually be caused by an underlying joint problem." If you haven't been referred by a medical practitioner, he recommends a check-up by a chiropractor or other expert before embarking on a Pilates course.

But Pilates can teach you how to isolate and use specific muscles properly and therefore help protect against doing inadvisable things with them as you - literally - move through life. "Someone may be able to touch their toes," Hutchful offers as an example, "but if that gross movement isn't evenly distributed among all the parts that are supposed to bear the load - calf muscles, hamstrings, hips, lumbar spine and so on - they can still have back problems. Pilates can teach you how to use each part properly."

The Alexander technique

This is a more fundamental approach to good posture than either Pilates or yoga. It aims to teach people to regain the natural poise we all had as children. "It's about unlearning bad habits and giving you a gentle awareness, at the back of your mind all the time, about the right, loose, free, natural way we all used to do things at one point in our lives," says Noel Kingsley, Alexander teacher and author of Perfect Poise, Perfect Life, as he repositions my head with one hand, hips with another.

The experts have minimal beef with the Alexander technique, although Hutchful points out that it will be of little help to those with joint problems, which Alexander teachers are not (and do not profess to be) able to work on.