Jeanne Day greets the morning cheerfully. Up by 6.30am, she pulls on her tracksuit and heads out in to the bracing Fife air to walk a couple of miles over St Andrews' farmland. Then it's back home for some meditation, floor exercises and a breakfast of fruit. Between 9am and 6.15pm, she is available to teach but if she's not fully booked, she'll take in another long walk or do a spot of gardening, followed by a light supper and more gentle exercises on a cross-trainer. After 8pm, she's often on the phone with family and friends, or keeping up with her emails.

Nothing extraordinary about this - it's the sort of lifestyle many of us lead. But Day is 87 years old. How does she do it? "I've always been what I call 'a cheerful chappy'," she says. "I'm not a complainer. I don't say 'Why me?' but take things as they come and see what I can learn from it."

It's all very well being a happy-go-lucky person, but that doesn't account for the fact that Day looks and acts well over a decade younger than her years. Slim, sprightly, mentally alert: what the multi-million-pound anti-ageing industry wouldn't do to bottle some of this elixir. So what's her secret?

Though Day eats healthily - no meat and about 80% raw food - it is not just her diet that keeps her sprightly, but her devotion to the Alexander Technique (AT), which she has been practising and teaching for nearly 50 years.

A tricky thing to describe, AT is not like any other fitness or health craze; neither a therapy nor a fitness regime. It's described as a simple and practical method for self-improvement and self-help, taught in one-to-one lessons with a qualified teacher. Your goal is to become more aware of how you have been "misusing" your body. Popular with musicians and actors, it is good, too, for city workers who sit scrunched up over a desk with a phone jammed under their chin. Although it's never become a craze on a par with Spinning or step aerobics, plenty of well-known names are aficionados, from John Cleese and Paul Newman to Fergie, Duchess of York.

Day is certainly a living exponent. Her body is flexible and she walks with a calm grace. Perhaps she has always been like this, but AT seems to give her superhuman energy levels too. When she turned 80, to make sure her mind stayed alert, she decided to take on a new activity every year. First it was scuba diving, in the local swimming pool with an instructor. "I got my certificate, but wasn't that keen on all those tubes and underwater breathing. So then I turned to t'ai chi and I love that, it's part of my morning routine. The next year, I decided to learn Esperanto and I've never stopped. It's a fantastic language and recently I came out with a distinction, passing the exam with 83%."

This summer, Day will be going to an Esperanto summer school. By the way, she also translates the works of the philosopher Martinus from the Danish.

But teaching AT remains her day job, at an age when most people would have been long pensioned off. Day was one of the pioneers of this specialised method of health "re-education", developed in the 1890s by an Australian actor, F M Alexander, and carried on by Walter Carrington in London in the 1960s. Day trained with Carrington, as one of his early students at the Holland Park Constructive Teaching Centre.

"I was already 41 when I discovered the Alexander Technique," she says. "I'd had various jobs, done all sorts of things, but not found what I wanted to do in life. At the time, marriage and children weren't for me. Without the Alexander Technique, I'm sure I would have developed problems with my muscles and joints."

So what does having a lesson feel like? There's only one way to find out. Day guides me in a new way to stand from sitting, and to sit from standing. My experience of standing up from a sitting position is nothing like I have ever experienced before. I am not pulled up. Nor do I clench my leg or buttock muscles, my neck or back. Almost as though by hypnosis, I find myself lifting out of the chair, with Day's hands guiding me forward. In another part of the lesson, on the table, she encourages me to lengthen out my legs one by one. Her hand cradles one heel of an outstretched leg and, wow, I can feel my leg involuntarily moving towards her. It's an amazing sensation.

But how am I going to change my bad ways? Like most people, I'm so used to pulling my back up straight, jaw stiffened, to meet deadlines or get things done. The trick is not to take home your list of exercises or things "not to do". Day explains: "One of the greatest things is to learn to 'stop'; we have to stop reacting to the stimulus 'to do'. When I was guiding you out of the chair, you had to stop your mind from pressing the 'I'm going to stand button'. Your head, given its freedom, will lead the way."

· Your local Alexander Technique teacher can be found at www.stat.org.uk