Like drinking games, but a bit better for you.
Anyway, I find that assigning myself exercise directly proportional to how much I fail in a game helps to both improve my game and get me off my rear end.
Example: For every point I miss out of 100 in a Yoshi's Island level, I do one crunch. I'm not great at getting points in that game, so a decent workout every two levels or so (or even one level, if I do really horribly).
Anyone else do this? It's not difficult at all (well, not planning it, at least). Just find something quantifiable in a video game and tie it to an exercise that you can do.