A bit of strategy can help.
What Could Homer (Or You) Do?
1. Get 2 meals out of it. Ask for a to go bag when you order your food. Portions are almost always larger than your actual needs.
2. Drink water with your meal. You will realize you're full sooner, and we need water for every day metabolic activity.
3. Vary your food groups. Order a side of spinach in addition to the steak.
4. Invite Bart or Marge to join. Conversation will slow you down, allowing your body the 30 minutes it takes to signal to you that you're full. Listen to what your body is telling you.
5. Order side dishes instead of an entree'. A vegetable side + soup + protein side = reasonable portions . . . this will also let you try more foods.
6. NO is a complete answer. When the waiter asks if you want more of something, simply say "no". You don't need to explain why.
7. Be different. If your friends order double cheese burgers, it doesn't mean you have to. When you confidently ask for a turkey burger, and hold the special sauce, they might make fun of you. Who cares, because the next time, your friends might copy you.
Restaurant food is generally higher in salt and calories, and tends to offer less "nutrient dense" foods (ex : fruits, vegetables and nuts). The owner wants you as a customer and the easiest way to do that is to make certain the food tastes good. So, they use more oil and add extras like breads, chips and sauces - but when it comes down to it - you likely don't cook this way at home. If you slow down, and practice paying attention to your body, you might find you don't need those extra items to be satisfied by the meal.