No Snacks, no sweets, no seconds. Except on Days that start with S. Too simple for you? Simple is why it works. Look here for questions, introductions, support, success stories.
"In the case of the French paradox, it may not be the dietary nutrients that keep the French healthy (lots of saturated fat and alcohol?!) so much as the dietary habits: small portions, no seconds or snacking, communal meals — and the serious pleasure taken in eating. (Worrying about diet can’t possibly be good for you.) Let culture be your guide, not science."
Michael Pollan's stuff is always good, but this article is especially worth reading because it's not about huge societal forces beyond our control, but simple, small, obvious things we can do right now. Thanks for the link! (and the very relevant quote).