Usually music, occassionally I'll hear someone say something that indirectly lights a fire in me. I also used to do a lot of writing so if I'm feeling a bit down I'll look back on some of my old work and smile.
Basically, I've learned that motivation and happiness is something that has to come from within you. Sometimes you just have to put yourself first and make yourself happy no matter whether other people approve of what you're doing or not. You can't expect other people to give you anything. Do your own thing and don't try to model or criticise your own life based on what someone else has.
Maybe I'm being vague, but that's usually what I tell myself when I feel I need a boot in the ass.