Let's see... Avoid novelty; Only take on easy things; Don't think creatively; Do things the most convenient way; Don't network.

...in other words, live a stationary life, do the same things you always did in the same way with the same people.

I don't know if any research exists to support this but it seems pretty likely, to me, that this will make one less intelligent.

I think brain can be thought to behave like a muscle and skeleton. If you put it under stress(learning) and let it recover(sleeping) it improves and grows stronger. Similarly if you let it idle, atrophy hits and it weakens. Just like muscles and bones do. Both can be trained with consistent and proper effort.