The first afternoon is fine. You might feel a little breathless after zooming down the sun-kissed piste like a latter-day Franz Klammer, but there's a warm mug of something waiting in the chalet and an evening of après-ski to look forward to. By the second day your legs are starting to feel the strain. By the third they are burning, almost - it feels - literally. That's right, you've just gone and underestimated what is required to get through a holiday in the mountains. And it can turn a dreamy week away from the British winter into a stiff, achey nightmare.

"In the past people have tended to underestimate how fit you need to be, but they're starting to get the message now," says Graham Bell, who skied downhill for Great Britain in five winter Olympics and now presents the BBC's ever-popular Ski Sunday programme. "Skiing is a very physically intensive sport - more so than anything else that anyone does on a regular basis, even something like squash. The longest run down a slope most people can do without stopping is two to three minutes. The longest descent I ever did was off-piste and it took six minutes. We did it twice and the next day I had to crawl down the stairs."

To avoid a similarly undignified descent to the hotel dining-room every morning it is important that you start getting into shape about six to eight weeks before you head off. And since most Britons take skiing holidays in February, the time to begin is approaching fast.

Betony Garner, a spokesperson for the Ski Club of Great Britain, says there are three main areas to concentrate on: cardiovascular, leg and core strength, and suppleness. Cardio-vascular training is the unsung hero of physical conditioning. It requires lots of work behind the scenes, but for a sport like skiing - where bursts of activity often follow hot on the heels of a period of rest, such as sitting on a ski-lift or enjoying a leisurely lunch in a restaurant on the slopes - it is crucial for preventing tiredness, which can lead to injuries. At high altitude, this becomes doubly important.

"You should attempt some type of exercise three times a week for a minimum of 20 minutes each time," she says. "More if possible, but you should always consult a doctor first if you don't do exercise frequently. Cycling is one of the exercises that best replicates what you do when you ski, and if you have access to a gym the cross-trainer is very good too."

Then there are the legs. Anyone who has seen an international downhill skier cannot fail to goggle at the improbably chunky thighs. Antoine Dénériaz, who won gold in the men's downhill at the Turin Olympics earlier this year, weighs in at 115kg and Bell compares the standard physique to that of a rugby player. This is not to suggest you embark on an insane bench-pressing frenzy, but rather to emphasise the importance of preparing your legs for a bit of a buffeting.

"The legs are key," says Garner. "A lot of people don't have the leg strength to cope properly, which can mean injury and will definitely mean you won't enjoy it. Lunges and squats are very useful for helping the hamstrings and quads, which are the crucial muscles involved in skiing, along with the backside. Single-leg squats are a step up from normal squats, and if you're fitter you can try what we call dynamic lunges, where you stick one leg out in front of you and then jump with the other. Make sure the knee isn't pushed out in front of the foot: the leg should be at a 90-degree angle. Try to do three sets of 20 lunges every other day. You need to give the legs some rest between sessions."

Garner also advocates working on your core stability: stomach muscles get used a surprising amount on the piste, especially if you keep falling over. "Simple things like hopping can help," she says. "You can try triceps dips off sofas or sit-ups while watching EastEnders. The plank exercise - lying on your forearms - is very good for core stability, and simple crunches are good, but not the full sit-up. Think about pulling your stomach muscles in towards your back - it works them harder, whereas a normal sit-up can be concentrated on the outer muscles."

If you are wondering why flexibility and suppleness are important to skiers, then imagine trying to keep your balance when one of your skis hits an errant lump of snow or ice halfway down a steep slope. "What sets downhill skiers apart is their balance and coordination," says Bell. "A top-class skier could ride a unicycle and juggle at the same time or stand on a wobble board for as long as they like."

"It's important to warm up first and do your stretches," says Garner. "When you've warmed up, that's when you can develop your flexibility. Don't stretch from cold - run around a bit. If you've got the time, Pilates and yoga are great."

Getting fit for the Alps, in other words, does not necessarily mean spending money and time seeking out your nearest dry-ski slope, although the Skier's Edge machine - a piece of ski-specific technology which is used by the British team and has sold well to home users - allows you to recreate the side-to-side motion you find on the slopes. But the most important message is to work steadily. "It's better to exercise longer, more slowly and more regularly than to kill yourself straight away," says Bell.

And if that doesn't sound half as enticing as the act of conquering the piste, then it is an investment which will allow you to make the most of your holiday. "The classic injury is the torn anterior cruciate ligament [which connects the bones of the knee joint]," says Bell. "It can be down to a tired fall at the end of the day, and I've even seen people tear one after falling over while standing still. If it's a bad tear you'll need an operation and you won't ski again for six months. If you're fitter and your muscles are stronger, it's less likely to happen to you" ·