Meal size at restaurants are usually large, and the portion is usually more than you can eat, especially if you are out with less people. Further, people usually order two or more food items for variety. With so much delicious food on the table in front of your eyes, you may be more vulnerable to overeat.
In such a case, ordering a to-go box can help. Ask for one as soon as your food comes and transfer the extra portion in the box. Eat only as much as you need, based on your hunger and appetite. Saving extra food in advance will prevent you from overeating later.