Have you noticed how, since the Olympics, the parks, pools and gyms have been packed with people huffing and puffing their way through new fitness activities? Even ordinarily niche velodromes have become oversubscribed. There's no doubt about it: team GB's unsurpassed medal success has inspired many into action.

However, while charging into a new sport with all guns blazing is to be applauded, 40% of novices fall off their fitness wagon within the first year of taking them up, according to a Fitness Industry Association report. Professor Greg Whyte, from the Research Institute for Sport and Exercise Sciences at Liverpool John Moores University and author of Get Fit, Not Fat (Kyle Cathie, £14.99), urges proceeding with caution. "Don't expect too much, too soon, or you'll lose motivation," he says. "Bear in mind that the success of our Olympic athletes is the end product of years of sport-specific training. It takes time - not to mention hard work and commitment - to acquire new skills and undergo the necessary physiological adaptations required to perform at your peak. Be patient.'

According to Ralph Hydes, a London-based personal trainer who specialises in triathlon coaching, the first task is to set yourself appropriate goals. "A goal acts as a deadline, from which you can work backwards, putting in some 'mini' targets to achieve on the way," he says. "This gives you something to aim for and helps you stay motivated." Hyde believes many of us don't achieve our fitness goals because they are unrealistic - in terms of how challenging they are or how quickly we want to achieve them - or too vague. "You might say, for example, 'I want to lose weight,' but how much do you want to lose, and by when?' he says. "A clearly defined aim gives you a far greater chance of success."

But whether you have weight loss, a 10km race or a stage of the Tour de France in your sights, the golden rule for taking up a new sport is to go easy. "The commonest reason I see for people failing to maintain an exercise regime is that it is too challenging for their existing level of fitness," says Whyte. Overly demanding fitness programmes are likely to end in failure for two main reasons. Firstly, you're unlikely to stick to such a gruelling schedule, and even if you do hang in there for a while, you won't enjoy it much. Worse still, you risk overdoing it, getting injured and rendering yourself out of action.

Regular and consistent training is important, but that doesn't mean you need to be hitting the gym, track or pool seven days a week. "Only increase the intensity and amount of time you spend training gradually," advises Whyte. "In my experience, this is far more successful than attempting to go from inactivity to athlete overnight." Whyte should know. He coached actor David Walliams to swim the English Channel back in 2006. "He could just about manage a mile when I first started coaching him, but by making small but steady increases in his training, he was able to swim over 25 miles just 33 weeks later." Gradual, rather than drastic, increases to your training volume also make exercise more manageable, giving you time to integrate it into your life, which means that you are much more likely to maintain your regime in the long term.

But while you only need increase the level of your training a little at a time, it is crucial to keep upping the ante. If you continue to perform the same workouts week after week - without increasing either their intensity, duration or frequency - your body will become accustomed to them and your fitness improvements will plateau. "Failing to add an element of progression to training is a common beginner's mistake," says Hydes. "Aim to increase your overall training volume by around 10% each week for three weeks, and then take an easier week, during which you reduce the volume of training by 20-30%, before building again. This helps keep you physically and mentally fresh."

While motivation, goals and a little discipline are what it really takes to stay on the road to fitness, organisation and planning don't go amiss. A study on runners, published this week in the British Journal of Social Psychology, found that those who identified with the statements "I have made a detailed plan regarding when/where/how/how often to do my physical exercise", were the ones who increased their mileage the most over the course of a year in the buildup to a marathon. Follow suit by scheduling your workouts and keeping track of them in a training journal.

There is also growing evidence that spreading shorter bouts of physical activity through the day is an effective way of becoming active. For example, accumulating 30 minutes of moderate activity in, say, four five-minute bouts and a 10-minute bout. "The benefits are comparable to a single, longer session, but it's less daunting and more manageable," says Whyte.

Heeding all this advice will hopefully enable you to stick with your programme, and avoid the sports injury clinic. But if you do face a setback, or don't see results as quickly as you had hoped, don't lose heart.

The Beijing podiums would have seen far fewer British athletes if they had all thrown in the towel at the first sign of adversity. "It's important to see fitness as a lifelong commitment, not a quick fix," says Hydes. "It's only natural that there will be ups and downs, times when you progress faster or slower, but you have to try to stay focused." Achieve that, and I can't guarantee you'll have a gold medal around your neck, but you will be well on your way to achieving your fitness goals.