The brain may have billions of cells, but if we hope to stay sharp as we age, we need to use them or lose them. Actually, it’s not the cells themselves we lose but their ability to communicate with one another. “Memories are stored as connections between brain cells, and activities that stimulate the brain preserve those connections and help protect you from age-related memory loss,” says Jeff Victoroff, M.D., an associate professor of clinical neurology and psychiatry at the University of Southern California’s Keck School of Medicine and the author of Saving Your Brain (Bantam, $10 at www.amazon.com). What to do? Join a book club; learn a language; solve crossword puzzles. What else? Take up to 400 IUs a day of vitamin E, an antioxidant, advises Victoroff. Eat fish rich in omega-3 fatty acids, like sardines and salmon. Stop smoking and moderate your drinking. Finally, he says, “there’s good evidence that the more aerobically fit you are, the less memory loss you’ll have as you get older.” So you won’t be losing track of… What was that again?