Evidence that willpower can be trained - see also a comment by Unnamed.

You hear a lot of advice from high-willpower people that you should just try harder and you'd be like them. I think it's hard to support a claim bigger than "you'll get better at doing this type of task more efficiently" (something a lot more specific than "things requiring willpower"). I know that studies attempt to use a different post-intervention test, but they're usually so similar that the interpretations of the studies can be narrowed if you're not feeling generous.