Some specific activities include learning a new language, writing a short story or learning a new musical instrument, especially if none of those activities is something you do all the time. But most people don't have the time or opportunity to do that intense an activity, so Katz emphasizes everyday activities to change your routine.

"Try brushing your teeth with the opposite hand. Or take a new route to work," he says. "Any exercise that gives your brain a break from the routine is establishing vital new pathways and circuits among brain cells. The idea is to weave a more dense network of connections so if a few fray as you get older, you'll have others to fall back on."

He eschews tricks like memorizing poems or random lists to impress people with at dinner parties. "We are really emphasizing just overall kind of brain health and agility, being able to think creatively, feeling confident that you can undertake mental challenges, rather than being able to astound your friends with your prodigious memorization powers."

Dr. Amir Soas, of the Case Western Reserve University Medical School in Cleveland, says the more complex the task, the more synaptic activity is taking place. He believes that more cerebral activities like creating something, playing chess, learning a new language or doing a crossword puzzle, are more valuable.

"You're reading, you're thinking, you're writing, so there's more motor and sensory areas of the brain that are working simultaneously, so you have a greater number of synapses working simultaneously. And of course, like anything else, the more you use it, the better it becomes."

If the right kind of mental exercise promotes a healthy mind, the absence of mental stimulation may have an equally harmful effect on the brain, by allowing neurons to atrophy and die.

One of the worst things you can do for brain health is to watch a lot of television, Soas says. It isn't that TV by itself is so bad, because brain-wave studies have basically shown you go into neutral, almost a sleeping state. But other studies show TV watchers are sitting down eating high-fat, high-salt junk food and not exercising mind or body.

It's so bad that Case Western plans to study whether people who contract Alzheimer's watched more TV throughout life than healthy seniors.