How to have more self-discipline | Penelope Trunk's Brazen Careerist
http://blog.penelopetrunk.com/2009/07/08/how-to-have-more-self-discipline/

That is, if you can work hard to have self-discipline in one, small area, you create self-discipline almost effortlessly in other areas. The most famous study about this phenomena is from Baumeister, who found that students who walked with a book on their head to fix their posture ended up eating better, studying harder, and sleeping more. Without even noticing they were making those changes.

This all rings very true for me.