Do things you used to do when you were younger.
Exercise your brain.
Exercise your body - 30 minutes of walking / fast walking a day is all it takes.
Eat healthy.
Drink plenty of water.
Take your vitamins.
Relax / Meditate.
Sleep well.

After reading all these books about improving your mind as you age, those 8 tips are the gist of it. How you do it is up to you. The key thing is that you just start and incorporate all this into your daily life one small step at a time and make it a habit.