Many people will have heard of the Alexander technique but have only a vague idea what it is about. Until earlier this year, I didn't have the faintest idea about it. But, hunched over a computer screen one day, I noticed that the neck- and backache I regularly suffered were more painful than usual.

I consulted an osteopath, who said: "I can treat the symptoms by massaging your neck and upper back. But you actually have bad posture. That is what you need to get sorted out. Go off and learn the Alexander technique."

I had regularly been told by friends and family that I tend to slouch in chairs but had thought bad posture was something one was born with and could do nothing about. That is not true. Dentists and car mechanics, among others, tend to develop bad posture from leaning over patients or engine bays. Mothers often stress and strain their necks and backs lifting and carrying children, and those of us who sit in front of computers all day are almost certainly not doing our bodies any favours.

But we are born with excellent posture. Look at a young child playing with a toy on the ground. Invariably, he or she will squat with legs bent but back, neck and head in perfect upward alignment. Learning to rediscover that youthful excellence is perfectly possible and, it turns out, highly enjoyable.

A few clicks on the web and I found an Alexander technique teacher, Tanya Shoop, in my area of south London and booked a first appointment. Three months later I am walking straighter and sitting better while my neck and back pain are things of the past. I feel taller, too, which I may be imagining, but the technique can increase your height by up to 5cm (2in) if you were badly slumped beforehand.

The teaching centres on the neck, head and back. It trains you to use your body less harshly and to perform familiar movements and actions with less effort. There is very little effort in the lessons themselves, which sets apart the Alexander technique from pilates or yoga, which are exercise-based.

A typical lesson involves standing in front of a chair and learning to sit and stand with minimal effort. You spend some time lying on a bench with your knees bent to straighten the spine and relax your body while the teacher moves your arms and legs to train you to move them correctly.

The key is learning to break the bad habits accumulated over years. Try, for example, folding your arms the opposite way to normal. It feels odd, doesn't it? This is an example of a habit the body has formed which can be hard to break.

Many of us carry our heads too far back and tilted skywards. The technique teaches you to let go of the muscles holding the head back, allowing it to resume its natural place on the summit of our spines. The head weighs 4-6kg (10-12lb), so any misalignment can cause problems for the neck and body.

Alexander work teaches you to think of the space above your head. This may sound daft, but it is an important element in the process of learning to hold yourself upright. You learn to observe how you use your body and how others use theirs - usually badly. Look how a colleague slumps back in a chair with his or her legs crossed. That puts all sorts of stresses and strains on the body. Even swimming can harm the neck. The Alexander technique can teach you to swim better, concentrating on technique rather than clocking up lengths. "In too many of our activities we concentrate on how we get to a destination rather than the means or way of getting there," says Shoop.

Teachers spend three years in training. The average person usually requires about 30 lessons to get a good grounding in the technique. Each lesson costs between £25 and £40. In terms of an investment in our bodies, it's cheap.

In addition to lessons, you are encouraged to spend five to 10 minutes a day lying semi-supine - on the back with legs bent up, head propped on a few books and hands resting on the chest. This widens and lengthens the spine, helping it to return to its natural alignment.

So who was Alexander and how did he come up with the technique? Frederick Matthias Alexander, an Australian theatrical orator born in 1869, found in his youth that his voice was failing during performances. He analysed himself and realised his posture was bad. He worked on improving it, with dramatic results.

He brought his technique to London 100 years ago this month and quickly gathered a following which included Aldous Huxley, George Bernard Shaw and the politician Sir Stafford Cripps. He died in 1955, having established a teacher- training school in London, which is thriving today.

So if you are slouching along the road one day, feeling weighed down by your troubles, give a thought to the Alexander technique. It could help you walk tall again.

Five back tips

1 Lie semi-supine for 15 minutes daily (on your back, head on two or three books, hands on chest and knees raised).

2 Try to think of the space above your head as often as possible, especially when sitting or standing, jogging or walking.

3 If you are gritting your teeth or have your tongue pressed to the roof of your mouth, allow the jaw and tongue to soften.

4 Stop before you lift anything and think how to use the least effort.

5 If you swim, particularly breaststroke, explore how to do the stroke properly, breathing out underwater, to minimise stress on the neck.

· To find out more about the Alexander technique or the location of your nearest teacher, the Society of Teachers of the Alexander Technique can be found at www.stat.org.uk