Behaviour modification techniques such as "self-talk", which encourages the belief that you're already living a healthy life rather than being on the way to one, have long been used in other areas of behaviour counselling, including smoking cessation, weight loss and sports performance, but they're only now finding their way into the personal training market.

"People are much better informed about exercise these days, but what they struggle with is how to incorporate healthy living into their daily routine in the long term," Archer believes.

That doesn't mean trying to terrify them into it. In fact, research from Sheffield University found that using scare tactics was the least effective strategy in instigating health behaviour changes. So what did work? Setting goals, using self-talk, behavioural contracts and regular monitoring.

That's why one of the first things a personal trainer will do when you hire them is determine what you want to get out of working with them. "Many people embark on exercise with no goals at all, or just vaguely formulated ideas such as 'tone up' or 'get fitter'," says Cohen. "Helping clients pin down exactly what they want to achieve is crucial." Research from Aberdeen University found goal-setting resulted in significantly more weight loss than not setting goals. "The same is almost certainly true with exercise," says Cohen.

So what makes a good goal? "One that's challenging but achievable," says mental performance and lifestyle coach Midgie Thompson. Once you've determined your ultimate goal, you need to work backwards from there, setting mini goals along the way, which will boost morale and keep you on track.

Putting your goals in writing is widely recommended. "It creates accountability - a powerful incentive," explains Thompson. But be careful what you write. "The language you use to state your goal is important. I encourage clients to set 'towards', not 'away from' goals. For example, rather than saying 'I want to lose weight', I'll get them to say 'I want to reach my ideal weight.' The unconscious mind doesn't like 'losing' anything - so the way you frame your goal needs to be positive."

Unfortunately, most of us feel anything but positive about physical activity. In many ways, exercise is seen as punitive; as soon as you start telling yourself you "have to" lose weight, it's all about negative emotions.

So what can a personal trainer do to change things? Stephen Kraus, a US-based scientist who runs a service called Psychology for Fitness Professionals, believes that while trainers shouldn't play the role of psychotherapist to their clients, an understanding of human behaviour, and some of the science-based tools used to modify it, is important.

One technique Kraus recommends is "root cause analysis"; in other words, you ask the question "why?". Why do you want to take on a personal trainer? Because I want to lose weight. Why do you want to lose weight? So I'll have more energy. Why do you want to have more energy? So I can play with my children. "This technique helps to clarify motivation and purpose, and makes it much easier for you - or your trainer - to keep on track when the going gets tough," he says.

But what if the thing that motivates you to lace up those trainers is shamefully shallow - does it matter? Research on "intrinsic" and "extrinsic" styles of motivation suggests it does. Studies show that extrinsic factors - like losing weight for your wedding or holiday - are associated with short-term commitment, while intrinsic factors - say, the sense of accomplishment you feel from getting fitter, or the enjoyment of playing a sport - are associated with long-term adherence.

Then again, part of the battle with exercise is making it a habit - so who cares if those first few sessions are motivated by nothing more lofty than attaining a six-pack? "In my view, it doesn't matter whether your motivation is deep and meaningful or superficial - as long as it gets you out there," says Thompson.

And, according to Cohen, if you do manage to get through those first few weeks, you'll begin to develop the neurological pathways that make exercise feel "normal". "Experts estimate that if you can sustain the habit for 21 sessions, it will become routine," he says. And that, of course, has to be the ultimate goal.

How to find the athlete within
Steps to mental fitness

· What are your reasons for exercise? Write them down, in as much detail as possible.

· Set challenging but achievable goals. Establish your ultimate goal, then work back to the present, putting in nearer-term goals to achieve along the way. Put your goals into a time-scale and have them in writing.

· Learn to self-talk. Everyone has times when they don't want to exercise, but the ones who do it anyway are those who have learned to counter negative self-talk with positive, persuasive arguments.

· Try to stay "in the moment" during your work-outs, rather than switching off completely, or thinking about other things. Feel the breeze on your face when you're out running, or tune into your breathing, rather than focusing entirely on the numbers on your heart rate or pace monitor.

· Don't get stuck in a rut. "Have a sense of discovery and fun about exercise," says Cohen. "You've got to enjoy the journey, not just the destination."

· Use visualisation: picture yourself on the treadmill or taking part in that kickboxing class.

· Post-workout, take a moment to congratulate yourself - and reflect on what you've achieved.