I don't really diet. I try to eat healthy, but if I *really* want pizza or sweets, I'll indulge. I HATE overeating--it makes me feel so nauseated afterward, so if I'm tempted to eat an extra slice of cake or something, I just think of what my body would feel like afterward. I do make exceptions for special occasions - Thanksgiving, anyone? - but it usually works.
Also, I've found that, generally, the less junk food I eat, the less I want to eat. Again, it just feels so much better to eat healthy food!