The more you use your brain, the better it'll function. Often-used pathways are reinforced, while seldom-used ones are degraded. Keep learning, and practise the things that you've already learned. If you're not ambidextrous, try to learn how to do things opposite-handed. Number and word puzzles, along with playing an instrument are excellent mental exercise. Combined with a healthy diet, including having enough essential fatty acids, trace nutrients etc..., and you'll be doing well.
Jamesmartin: you would probably benefit more from therapy than taking more pills. Fish oil is awesome, but it's not really an antidepressant. Omega-3s are excellent neuroprotective and neuroregenerative agents, but I'd personally prefer to get them from plant sources. YMMV, of course.