How to give your brain a workout:

Try writing backwards or use your non-dominant hand. This creates a different neural pathway,similar to when you get new ideas. Da Vinci practiced mirror writing to keep his/her brain fresh.
Chew gum: This increase activity in the hippo campus, the part of the brain important for short term memory.
Learn a second language, forcing your brain to continuously switch tarcks. This is one of the most mentally demanding things we can do.
Watch less television
Eat food high in Omega 3 fatty acids such as berries, seeds, turkey and fish.