I read someplace that discipline is weighing what you want ultimately vs. what you want *now*. I try to remember that when I'm filling out applications, I want to write 4.0, not 3.5, and the difference between the two may very well be whether I get out of bed and go to class today. Everything adds up, and everything matters. My future excellence is a product of what I do this minute and the next minute and the next minute.
It doesn't always work - I like naps as much as the next person - but it helps motivate me most of the time.