I found this quote on the always wonderful EconLog. In discussing the Flynn Effect, James Flynn writes:
There is one way an individual can walk a personal path to enhanced cognitive skills. He or she must internalize the goal of seeking challenging cognitive environments -- seeking intellectual challenges all the way from choosing the right leisure activities to wanting to marry someone who is intellectually stimulating. Better off still are those who develop a certain kind of character formation -- a character such that I carry about within myself a stimulating mental environment I myself create. Then I would be relatively free of needing good luck to enjoy a cognitively enriched environment throughout life. I would have instant access to a portable gymnasium that exercises the mind. Books and ideas and conversation are easier to transport than a basketball court. No one can keep me from using mental arithmetic so habitually that my arithmetical skills survive.
I love this quote on so many levels. Put concisely, one has to put himself in a position to exercise the mind. If not, just like the body without exercise, the mind turns to mush.