As we live ever longer, people with severe memory loss and dementias such as Alzheimer's, are becoming more common. "We're all going to get Alzheimer's if we do nothing to protect our brains," says Gary Small, director of the centre on ageing at the University of California, Los Angeles. In the UK, some 750,000 people have Alzheimer's, a number expected to rise to 1.5 million by 2050.

"It's not just about memory loss," says Richard Harvey of the Alzheimer's Society. "Ultimately you are dependent on others for everything from eating to getting dressed." The good news, as Small suggests, is that there are things we can do to protect our brains, to put off the inevitable.

There is little we can do about what we were born with. The luck of the genetic draw is reckoned to contribute about a third to whether or not we will develop Alzheimer's. About 20% of the population have a gene called Apoe 4. They have a higher risk of developing Alzheimer's and developing it earlier in life. But the gene is only a risk factor, increasing the chances of getting the disease. A full two-thirds of the risk of developing Alzheimer's is down to environmental factors - what we do to our bodies, what we don't do, and what we put in them.

Regardless of how we treat our brains, we can only mitigate the ageing processes that lead to memory decline and dementia. Two processes in particular contribute to our gradual slide towards a state that would be clinically defined as Alzheimer's. The blood vessels in our brains begin to stiffen up, a process accelerated by high blood pressure. Not only does this impair the flow of blood around the brain, it also prevents blood, with its valuable oxygen cargo, passing through the capillary walls to nourish the brain cells that need it.

"It makes the whole process much less efficient," says Roger Bullock, director of the Kingshill Research Centre, in Swindon, that specialises in dementias. "If they stop getting as much oxygen, they start dying."

The second process is more contro versial. The brains of Alzheimer's patients are peppered with clumps of a protein fragment called amyloid. These clumps, known as plaques, are thought by some to cause Alzheimer's, but their precise role has not been nailed down.

"Amyloid is definitely toxic to the neurons involved in memory," says Bullock. But many researchers think amyloid may be a more or less harmless result of the disease rather than the cause. Amyloid is just a fragment of a larger protein that is vital for creating long-term memories.

"The intact protein strengthens the links between neurons responsible for memories," says Steven Rose, director of the brain and behaviour group at the Open University. He believes that Alzheimer's is caused when the amyloid fragment breaks off the protein, damaging its ability to help form memories. Rose is due to announce a new potential therapy for Alzheimer's at the Memory International conference to be held at the Commonwealth Institute in London on July 9. He has found that injecting or feeding a fragment of the unbroken protein to animals not only prevents memory loss, but helps improve faded memories. So far, it has been shown to work in chicks, rats and mice.

Amyloid plaques alone might not be enough to cause Alzheimer's though. A study of aged nuns carried out in the US in 1997 revealed that while many of them had clinical signs of Alzheimer's - dense specks of amyloid in their brains - few showed obvious signs of mental impairment. The researchers found that those who did show clear signs of Alzheimer's had also suffered at least one minor stroke in their lifetime. "It's probably the combination of the two, strokes and plaques, that tips the condition into dementia," says Bullock. Unlike severe strokes, small strokes can easily happen without the person knowing. "If you get just a small amount of bleeding in the brain, you might have an off day, but you won't get any physical problem from it. You'd never know," says Bullock. "The trouble with the brain is that it's got no pain fibres. If it did, the last 20 years of life would be pretty uncomfortable."

Alzheimer's becomes far more prevalent past the age of 65, when the risk is only about 5%. By 85 the risk rises between 30% and 50%. The signs are often evident much earlier though. "It starts, probably from mid-40s, which is worrying," says Bullock. "It doesn't just come on and nab you."

The gradual decline in memory is often obvious. "You get a slowness of retrieval, the tip-of-the-tongue phenomenon which you start seeing in people in their 30s," says Small. As people enter their 40s, evidence of areas of dead brain cells and then plaques begin showing up on scans. Everyone has occasional lapses when, on wandering off to get something, you forget which room you were heading for, says Bullock. "We all do it sometimes and we joke about it, but these are snapshots of just how fragile our memory is."

We shouldn't just sit and wait for dementia to claim us, says Small. He advocates brain exercises to help keep the brain young and, inevitably, the sooner we get going on them, the better. One of the best preventative measures is to exercise our brains while we are still at school and continue to do so when we start work. "If you look at who gets Alzheimer's, you see that people who were more successful in education and have more mentally active jobs have a lower risk," he says.

Work your brain when you are young and you build up more brain-structure connections between neurons that represent memories. As ageing takes its toll, you then have more to lose before your memory becomes severely impaired. Conversely, neglect your brain and dementia will likely creep on earlier, as even a small loss of brain function could be enough to cause noticeable impairment.

Small suggests even simple exercises can help. If you are right-handed, try using your left hand more; try crosswords; learn a language. If none of that appeals, there's always the PlayStation. "Get someone on a computer game and their brain will work very hard at it at first, but after a while, you see less brain activity. It means their brain is getting more efficient. That suggests that if we work our brains out, we get better efficiency and it protects them," he says. The important thing is to ensure the exercise is novel, interesting and enjoyable. "Otherwise, it'll be too stressful and there is a whole load of evidence showing stress is not good for the brain," he says. Stress releases a hormone called cortisol which also kills brain cells. If you are stressed, you are far more likely not to remember things.

Small also advocates a "brain diet" which cuts down on all the things that are unhealthy for the heart. "What's good for the heart is good for the brain," he says. Small recommends foods high in antioxidants, such as berries, broccoli and prunes, and fish oils for omega-3 fats. Calorie-watching is also important as being overweight often goes hand in hand with diabetes and high blood pressure, which increases the risk for developing all types of dementia. One problem is that only about a third of the people with high blood pressure in the UK have been diagnosed.

In moderation, alcohol has been shown to be good for the brain. Four units a day, the equivalent of two pints of lager, is better for the brain than being teetotal. It has to be little and often though: binge-drinking a week's quota in one sitting is no good. Cigarettes also help memory, by stimulating receptors in the brain that are sensitive to nicotine and just happen to be crucial to memory. "I wouldn't advocate smoking though, because it kills you," says Bullock.

Simple exercises and diets can help stave off memory loss, but ageing isn't the only reason we are sometimes so poor at remembering. "One of the biggest reasons people don't remember things is that they weren't paying attention in the first place," says Small. For people who find themselves struggling to remember associations, like names and faces, he recommends a system he calls "look, snap, connect". The look part involves focusing on a defining feature of the face, such as prominent ears or nose, and making a mental image of it. The name should then be mentally linked to the image by creating a scenario with another word or a person with the same name. To link two unrelated words, such as rhino and windowpane, Small suggests visualising a rhino charging along and crashing through a windowpane.

Reassuringly, Bullock says that people who are convinced they have dementia often don't. "We had a guy who was drinking around 15 pints a day and drinking loads of coffee and smoking his head off. We put him on decaff, cut his drinking down and destressed him a bit and his memory was fine after a while. Beforehand he was really suffering."