Go out and buy a book called "The Brain That Changes Itself" by Dr. Norman Doidge. Read it to get a decent understanding of the latest neuroscience advances and understanding of neuroplasticity. Neurons do regenerate, the brain does not have to degenerate into mush as we age, but just as with physical exercise you need mental exercise. This area is still fairly new and advances are made all the time, unfortunately there is no agreed upon routine (so to speak) for brain health except that your brain adapts to different stimuli. Read, vary your subjects, increase your knowledge, turn off the TV, pick up books, do research, play games that stimulate the mind (as a poster above has stated). Think of the brain as a muscle that needs training, after a while our bodies become accustomed to a certain amount of training and we switch things up. Your brain works in the same way, switch things up, challenge your brain constantly and make it adapt to novel stimuli.
There is a tremendous amount of fascinating research going on right now and our understanding of neuroscience is really taking off. A google search will get you on your way re; brain plasticity, lots of authors you can look up, Norman Doidge, VS Ramachandran, Oliver Sacks, Paul Bach y Rita, Merzenich are a few of the pioneers/leaders in the field.
Muscles don't come in a bottle and neither do any of the characteristics of intelligence. Anything that is worth anything, takes work to achieve.