It’s much easier to decide before you start eating how much you’re going to have. If you’re trying to decide how many more to eat as you’re eating, it’s much harder to make a good decision. 

Here’s a great Response Card one of our dieters made about this issue:
Toss excess portion of whatever (ie-pasta) before I cook. When it comes to dipping crackers/such, create portion of crackers/dip before I eat. Eating is more enjoyable when I'm not worrying/obsessing about how many more I’m going to have.