from, "Food Rules: An Eater's Manual," by Michael Pollan
PART III: How Should I Eat? (Not too much.)
Rule #53 - Serve a proper portion and don't go back for seconds.
You lose all control over portion size when you have second helpings. So what is a proper portion? There is folklore offering some sensible rules of thumb based on your size. One adage says you should never eat a portion of animal protein bigger than your fist. Another says that you should eat no more food at a meal than would fit into the bowl formed by your hands when cupped together. If you are going to break the rule on seconds, at least wait several minutes before doing it: Yu may well discover you don't really need seconds, of if you do, not as much as you thought.
Eat food. Not too much. Mostly plants.