When you dine out, ask for a doggie bag container when you order and request that the waiter bring it when the food is served. Take half the food on your plate and put it in the container before you start eating. This saves you from the portion distortion that is prevalent in most restaurants.
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If you feel uncomfortable doing this up front, divide each item on your plate in half and put it on a separate plate to "keep it out of play." Then put it in the doggie bag at the end of the meal.