If you've ever suffered from or know someone with depression, you will know what a horrible illness it can be. Around 12 per cent of men and 20 per cent of women will suffer a major depression at some point in their lives. Of those, 15-39 per cent of cases may still be clinically depressed one year after the first symptoms. The cost in work time and impact on family and relationships can be huge. And once you've had one bout of depression, you're likely to have more.

Oxford University Professor of Clinical Psychology Mark Williams has been researching a technique known as mindfulness meditation, to help people who have had depression or suicidal thoughts to stay well. Based on the work of US-based psychologist Jon Kabatt-Zinn, in essence it synthesises insights gleaned from Eastern meditative traditions and cognitive therapy; teaching people to resist trying to problem-solve their way out of depression. Instead, through meditation, they learn to exist with their negative thoughts, observe them and in so doing detach themselves from their perceived problems, enabling them to step away from the spiral downwards into depression.

When we met at the Warneford Hospital in Headington, Mark explained how the brain is made to problem-solve with very efficient feedback systems. "It's designed to check in with how we're doing," he said. This works well with external situations, but when turned inwardly can be disastrous as people turn to rumination and self-blame. "Many people say, Why am I depressed? I shouldn't be depressed, I've got a great husband/wife/good house/good job. Why am I depressed?' The question actually makes their mood worse, so in two minutes time they check in and the feeling is even worse."

What mindfulness does is sidestep that loop with meditation. "By giving your mind just one thing to focus on - the body or the breath - then everything else can be seen and you begin to learn what a thought feels like. Normally we dwell inside our thoughts. This is actually seeing thoughts and observing them." When people have learnt to observe thoughts, then they can take action to change or live with them. Mindfulness meditation has been shown to cut relapse rates from 70-75 per cent to 35-40 per cent for people who have already suffered three or more episodes of depression. On the basis of this research, and that of fellow psychologist Dr John Teasdale, the National Institute for Clinical Excellence has approved it for use in the NHS for people with recurrent depression. Mark is now training psychologists, psychiatrists and GPs to run mindfulness courses and is setting up a master's degree in mindfulness-based cognitive therapy. "We're also hoping to set up an Oxford Mindfulness Centre, which will bring together the research and the clinical work, both for patients and the training work," he said.

Realising that the mindfulness techniques can only reach a fraction of people through courses, Mark has co-authored The Mindful Way through Depression with John Teasdale, Zindel Segal and Jon Kabat-Zinn. Together with a CD of guided meditations, it helps people who have suffered from depression or chronic unhappiness to learn what causes depression and how to avoid it.

Although this book is aimed at those with depression, mindfulness meditation can also be used to get the most out of life; to exist fully in what Mark calls the spaces in between.

"It's the spaces where we're waiting in a supermarket queue or driving somewhere, or we're always feeling we're not quite there yet," he said. "It's always next week that it'll feel better, when we get past this project, that's when I'll start enjoying myself. Sitting on a chair or walking up the corridor, this is also life and mindfulness can give us a sense of, okay, so we're actually alive right now. That can bring a profound change in people."

Mark came back to Oxford University five years ago, having lived here for nine years in the 1970s when he did his undergraduate degree and doctorate. How does he think the work he is doing now would have been perceived then? "My sense is that people were very wary of anything Eastern," he said. "They thought it was just a fringe activity done by esoteric people. I think it's become much more mainstream now, partly because the science justifies it. Evidence-based medicine is showing that some things that we might have thought of as being esoteric actually work very well."

I ask what he finds satisfying about his work. "It's the ability to combine work with people who have a long history of problems, working with them to see if we can help and research on how we can make it even better," he said.

He has what I can best describe as old-soul, peaceful eyes and an infectious, joyful laugh; he really inhabits the moment. A great advertisement for practising what he teaches.

The Mindful Way Through Depression: Freeing Yourself From Chronic Unhappiness is published by The Guilford Press at £12.99.