I've always prided myself on having a pretty good memory. In fact - without sounding smug - people have often remarked upon my knack for remembering minute details of long-past conversations, committing bank account and telephone numbers to memory or recalling the date and time of appointments without having to write them down. Lately, however, I've noticed things have been, well, a tad fuzzy. It's little things: like getting into a taxi only for my mind to go blank as to the destination, or opening a cupboard at home then immediately forgetting what I was looking for.

Then it hits me. In a typical week I spend at least 40 minutes each day walking, 20 minutes applying make-up, creams and potions and 10 minutes looking in the mirror contemplating the size of my hips, bum and stomach, yet all the while I'm neglecting what's inside - my brain. Sure, I read a lot and I'm always gleaning new facts and figures for my job as a writer, but it's been a long while since I gave my grey matter a really thorough work-out. It's time to take action.

For one week I will follow the steps in memory expert Tony Buzan's book Age-proof Your Brain. This involves spending an hour each day on exercises aimed at improving short-term, long-term and semantic memory. Tasks include general knowledge quizzes, memorising extensive lists and developing a new skill, such as playing a musical instrument. According to Buzan, I will also have to up my level of exercise, eat more healthily and kill stress in my life (note to editor: did you get that last one?). It sounds straightforward, but how much difference can it make in only seven days?

Before I even open the book, first up is changing my diet. Out is fast food, ready-made meals and anything high in sugar or salt. Instead, I stock up on vegetables, fish and fruit. Mid afternoon - when the sugar cravings usually kick in - I snack on nuts or a banana rather than head to the chocolate-bar-stocked vending machine. After a few days, I notice that late-afternoon fogginess, when my concentration tends to wane, has all but disappeared. My energy levels are up, too. Which brings me neatly to physical exercise: I'm not a fan of gyms, so instead I increase my daily walking regime from 40 minutes to an hour. This includes walking home from work every evening via an extended route. Cooking my healthy dinner of steamed salmon and vegetables, the exercise-induced endorphins begin to kick in and I'm all set for my hour of mental activity.

To be honest, I always thought puzzles were for geeks and the kind of freaks who still live at home with their parents when they're 47 (if ever I find myself sitting next to someone on a plane with a sudoku book in their hand, I raise my eyes heavenwards). Strangely, I surprise myself by finding I quite enjoy it. By the fourth day, I realise I'm actually looking forward to my daily dose of mental aerobics. Whereas on the first day, I ran over the allocated hour by a good few minutes, as the week progresses I find myself whizzing through the exercises. Memorising a list of 10 words? No problem. Popular culture quiz? A walk in the park. Recalling every prime minister since 1945? Easy.

Most notably, I'm beginning to notice I am getting better at remembering all life's important little details. While before I managed to forget on three consecutive days to make an appointment for a dental check-up, it's now not only made, but the date and time are stored firmly in my memory bank. When arranging to meet a friend for dinner at a new restaurant, I not only remember the name of the place first time but also the street and directions. How did I do this? Many of the exercises involve using Buzan's legendary mind-mapping technique: a thinking strategy and note-taking system invented by the memory expert around 30 years ago. In short, you use a simple image or symbol to represent your central idea, then add branches to this picture. It's successful because it mirrors the way the brain works. Other useful techniques I've gleaned are chunking (splitting telephone numbers into smaller, manageable groups of three or five) and patterns (for example the number 01019181945 includes the years the two world wars ended - 1918 and 1945 - an easy way to remember it).

To perfect my skills, I catch up with Buzan as he prepares for the World Memory Championships in Bahrain. He tells me anyone can train their brain to be like those of the memory grandmasters who take part in such events - able to perform impressive feats such as recalling a full deck of cards in less than 30 seconds or reciting a string of 188 numbers. He insists that a photographic memory or superhuman powers are not required.

"Every world memory champ has, at some time in their life, had acute poor memory and concentration," he says. "The grandmasters all train their brains in the same way that athletes train their bodies. It's true that with constant training, anyone can do those things."

Even so, I'm curious to find out why some people appear to have more impressive memory powers than others. "Some of us have simply had better training and nurturing, while many others have been trained unwittingly to forget," says Buzan. "The brain needs colour, image and association to remember. Brain fog is nothing to do with bad memory. It's because people don't build an image when they put down their keys or park their car. They don't connect the object to the environment around it, and therefore there is no image in their association, so the brain simply forgets."

Buzan maintains that anyone can age-proof their brain. "As the human brain gets older, on average its mental ability declines," he says. "That is totally unnecessary and not natural. The reason it declines is because people don't keep themselves physically fit, mentally fit or learn the basics of memory, which is imagination and association. They don't learn how to use any form of memory technique which is actually likely to be beneficial for their brain. These are all the things they need to do if their brain is not going to decline as they get older."

When it comes to physical exercise Buzan advises paying attention to four key areas: poise, aerobic training, flexibility and strength. Forms of exercise ideal for building these include walking, running, swimming, dancing, rowing and martial arts. A major study in Quebec, Canada, of 5000 men and women over 65 found that those who exercised regularly were less likely to show decline in their mental performance or develop Alzheimer's and similar diseases.

Research in San Francisco led by Kristine Yaffe, a professor of Psychiatry, Neurology and Epidemiology at the University of California, followed a group of 6000 women over eight years, monitoring their memory and other mental functions in comparison to their level of activity. Over the period of the study, those who walked least showed 50% more deterioration than those who walked most. Yaffe also discovered we can experience the benefits whatever our level of mental fitness. Every extra mile each woman walked in a week gave them 13% less chance of cognitive decline.

Healthy diet also plays a key part in mental fitness. "Every brain cell is made up primarily of the food you put in your mouth," says Buzan. "Good food, good brain; junk food, junk brain. Keeping your brain fit and healthy is as important as going to the gym. Regularly in life you need to be in both gyms: your physical gym and mental gym. Being sceptical is a waste of time. Try it and see if it works. Anyone who has tried these memory techniques has found they work within a day."