Several lines of research describe mechanisms in our bodies that cause a vicious cycle, so that when we overeat we are much more likely to continue to overeat. These mechanisms involve our biological clock, insulin metabolism, and nerve endings in the stomach. Even drinking too much water with a meal (especially ice water) can accelerate the problem. For more details, see this NPR story.
If you want to do only one thing to counteract your tendency to overeat, simply keep a journal and write down everything you put in your mouth. Food diaries or journals have been shown again and again to be effective tools in weight management. See these sites for more on food diaries (here and here).
For more information on avoiding overeating, see here.