We no longer tune into the same TV programmes at Christmas and have never been more different from each other, but many in Britain remain united by one thing: an obsession with physical perfection.

No time is this more obvious than the new year, when millions of dumpy Britons resolve to become thinner and trimmer in the months ahead. According to David Lloyd Leisure, starting a healthy diet and fitness regime is now the most popular new year's resolution.

Now that the turkey has been gobbled and the telly is getting back to normal, gyms are bracing themselves this week for the annual onslaught of recruits hoping to start the new year as they mean to continue.

But come February, around 30% of these well-intentioned conscripts won't even manage one visit to the gym a week, according to the Fitness Industry Association.

By the time 2008 rolls around, 40% of the original cohort will have ditched the gym altogether. At an average of £41 a month membership, locked in for the first year, it's an expensive resolution to abandon.

It is estimated that up to £200m is wasted each year in lapsed or unused membership fees.

January is by far the most hectic time of year for the Lancashire-based personal trainer Matthew Robinson - and also the most lucrative. "I could work every waking hour if I wanted to, and I did last year," he said.

Treadmills

"I get people coming in and signing up for 10 one-on-one sessions for £250, and I only ever see them once before they disappear off the face of the earth," Mr Robinson added. "My gym can hold over a thousand people, and for three weeks it will be absolutely rammed. There will be queues for the treadmills and no room to move. By the end of the month it will start tailing off, and come March it will be back to normal again. You get the odd one or two who do persevere, but they are the exception rather than the rule. Everyone else quits when they realise that the gym isn't the miraculous quick fix they were hoping it would be."

Graham Whittaker, membership consultant at the City Road branch of Fitness First, in London, said: "January and February are the busiest times of year for us. About 30% sign up for a three-month contract and only ever come once or twice. They do genuinely want to change something about themselves, but they lose motivation."

According to Howard de Souza, spokesman for the Fitness Industry Association, there are three main reasons people don't stick to their get-fit resolutions - and it's not a lack of willpower.

"Many people, especially working mothers, cite domestic and work commitments as being a reason for quitting, as well as the fact that often people choose the wrong gym," he said. "Perhaps it's near their work, when they only have a chance to go from home, or it's just got a lot of gym equipment but no pool, and what they really like doing is going swimming."

Detox

Justine Bold, a nutritional therapist, sees around a third more new clients at the beginning of the year wanting to lose weight. But she has noticed that these zealous new recruits are often not as committed as those who start with less fanfare later in the year.

"A significant proportion who come in now do not stick to a programme in the really long term," she said. "They have often been given a diet book for Christmas, or want to give up booze or chocolate or do a detox or some other fad for the new year."

And it is not just the health and fitness industry that benefits from this seasonal interest in wellbeing. Publishers and booksellers do very nicely out of it too.

"We see a huge surge of interest in diet, fitness and wellbeing books in January," said Fiona Allen, a spokeswoman for the Waterstone's book chain. "Publishers tend to bring out a raft of these titles at the end of December and at the very start of the new year to take advantage of this interest."

Pole dancing

Perennial favourites among the fitness and health titles include Mireille Guiliano's French Women Don't Get Fat: The Secret of Eating for Pleasure, Paul McKenna's I Can Make You Thin, Rosemary Conley's GI Jeans Diet, and anything written by Gillian McKeith, said Ms Allen.

But a number of more esoteric, new titles are expected to do well this January too, such as T Colin Campbell's "non-diet" book, The China Study, which is subtitled "the most comprehensive study of nutrition ever conducted".

At the other end of the spectrum is The Art of Pole Dancing: A Spin-by-Spin Guide, by Peekaboo Pole Dancin, proving that there really is something for everybody - and rubbishing the age-old excuse that "exercise just isn't for me".