I quit smoking after I stopped hanging out with people who smoke. Easier said than done in a lot of cases, but it really worked. This theory was proven when I started smoking again after moving in with a roommate who smokes. I'll probably quit again after our lease is up, for good this time.
That, and getting out there and going for a run, bike ride, etc. every day keeps me motivated to not pollute my lungs with crap.