Picked up a smart tip from a health magazine recently.

When you go to a sit-down restaurant that serves large portions, ask for a "to go" box at the start of your meal, rather than eating as much as you want and saving the leftovers. Put half the meal into the box, and eat the rest. It definitely helps restrain the urge to overeat, provides a tasty meal for later, and wins the battle over your mother's instructions to "clean your plate" that we all heard as children.

This is a great tip, thanks thrice!