Core strength is a *big* part of it. Most rides I conciously try to support my torso with my torso muscles rather than my arms. The more core strength I develop, the longer I can do that. (and the better I can do turn signals, and grab my waterbottle, and fiddle with my headlight...)

I think it's really misleading to say once you learn how to ride a bike you never forget... cause there's a lot of physical strength and skill needed that you *do* forget.