Use it or Lose it
You'll see ads for ginkgo biloba and other herbal remedies, but i've never seen good, double-blind studies that show efficacy from those products.
and substances, of course; excercise and sleep
well; and think, think, think. The more you use your mind, the better it will work. Read; play chess; do crossword puzzles-- less passive stuff like tv, and more active tasks.