everyone finds their own motivation. Maybe for you its fitting into soem clothes, maybe for others its looking at Jared the subway guy... to each his own.
beyond finding the desire, setting a schedule and not breaking it helps alot. As does getting a workout partner. Set reasonable goals. Dont start out saying you are going to go for an hour every day. Once you make it a routine, it will become easier to follow and harder to break. If you break the workout trend you will start feeling the negatives of not working out and want to go back. Not only will you feel fat and slow, you will miss the endorphin rush.
What works for me is playing sports. I'm well out of school and do not play team sports anymore, but I play tennis now and V-ball. I do not want to work out, but I do want to play sports. The good thing is once I get out and play sports, I dont mind, or even want to work out (in the gym).