Get competitive. I'll be honest with you, what I do is either have friends in classes that I can compete with, or I go after a girl in the class and use the fact that I'm getting better grades as a flirting point. Also it keeps me motivated due to the fact that I don't want to embarrass myself in front of said girl by getting bad grades.
Also, a steady diet of partying will allow you to recharge your batteries. And then once in a while going on autotrader and going thru the luxury cars and knowing I'll be driving one of those. Maybe going on Zillow and finding houses I can potentially live in.
The fact that I've told so many people that I'm going to be a doctor motivates me. I can't screw up, or I'll look like a man who has no word. All a man has is his word.