I find the only way I get willpower to not overeat is by slowing working up to it. I used to have to finish all the food on my plate, but by eating slower and drinking lots of water with food I realized I didn't have to. And the satisfaction I got from feeling in control slowly built up every time I either said no to food or ate just what I needed. Now I can pass up McDonalds or pizza, which is something I could have NEVER done before.
The feeling of eating to live and not living to eat is empowering :)