Quitting smoking.
I guess I'm lucky, I have a lot of friends who smoke (I'm 21) but cigarettes have always made me feel like ****... I will sometimes get an urge to smoke a cig but after I get halfway through one I feel nauseous and lightheaded and ask myself "why the hell was I doing this?" Now Mary Jane, that's a whole 'nother ball game lol...
Just think about your family, esp. kids if you have any... Imagine them growing up and puffing on 'cancer-sticks' all day long; for me that would be plenty of motivation but then again I haven't had to struggle quitting, I just told myself it's a total waste and think about how gross I feel every time I smoke.
Good luck, and keep at it!