When I get tired of doing school work and studying I just think of how crappy I will feel tomorrow when I have to do everything for the next day plus everything I didn't finish today! Or how good I will feel the next day when I have very little to do because I finished now!
And when I'm feeling just like it's all useless I think of how a career as a doctor is much more beneficial to EVERYONE than a career in retail. 🙂