My lifestyle and habits dictate that I don't gain much weight.
I'm scared of becoming like, obese, though. My aunt and uncle are uber-fat (think those two fat twins on the motorbikes, pretty much), and from what I saw, I think I'd rather die than be that way.
Anyway, it's just a matter of not overeating and having a productive diet. Exercise helps as well, but it has to be in combination with a healthy diet, otherwise you're not going to get anywhere.