When you know you are full, but can't resist "a couple extra bites" that may turn into a whole extra serving, put your fork, knife and plate in the sink. You can still sit at the table, enjoying water and conversation, but you are waaaay less likely to keep eating when you don't have utensils!
Discovered this after having ordered bad chinese take out. It wasn't even good, why did I keep eating it? After I put away my plate, I stopped eating!