I have two main sources of motivation.
1) The way I feel. I love to play basketball, go running, and be generally active, but it's so much less fun when I'm significantly overweight. So sometimes I just imagine what it'll be like when I lose it all and I can grab the rim, run a 5K really fast, or take up capoeira. I like to watch videoclips of insane athletes or just a basketball game or track event on t.v. That's always really good inspiration.
2) Having recently broken up with my long-time girlfriend, I want to look good so I can trick... er, convince another great woman to go out with me.

All I need to be motivated for this is see all the hot women walking around. And notice how differently women treat me now that I'm heavy. (Mostly, like I'm not even there.)