I make sure that my portions are all decided on and measured out before I ever take my first bite. This is easy at home, because you control how much you put on your own plate. However, I always make sure that the same is true when I eat out by either planning ahead and going to the restaurant's
web site and picking a meal that is completely within my calorie "budget" or by asking the server to bring me a box at the beginning of the meal so that I can box up the extra. There is a certain joy in knowing that I can, if I choose, eat "every last bite" on my plate without guilt.