Disclaimer: One meal out won't harm your progress. It is up to you to choose whether you want to stay on track or enjoy your meal. For the sake of this post however, lets say you want to stay on track.
1) If you know you are going out in the evening try and keep your calories through out the day a little lower. When you are out you can not predict how things are cooked, portion sizes etc. As a good way to stay within a good calorie range is to try and stick to lower carb and fat and higher protein meals through out the day. This should give you a bigger amount of leeway to guesstimate the calories in your food.
2) Ask for things how you want. Ask for no butter or oil when being cooked, or ask for sauces and dressings on the side. You can always change your sides for a lower calorie side such as boiled potatoes instead of chips or more veg/salad instead of a starchy carb. Don't be afraid to ask the waiter how your food is prepared, portion size etc.
3) Go for grilled over anything. A grilled piece of meat cooked in no butter or oil will be a great option for you. Moreover most restaurants offer grilled chicken, steak or fish. These will be great options to not only get in a decent amount of protein but to stick to healthier options.
4) If you are going to a restaurant check the menu before hand. This will give you a good indication of what you can eat and how you should eat through out the day.
5) Listen to your stomach. When you feel full don't feel like you have to finish your plate.
6) Very often salads can have a frightening amount of calories in them so scout out exactly whats in the salad. Many times grill piece of meat, veg and rice or potatoes could be a much healthier version.
7)Drink plenty of water before and through out the meal
8) Go for a plain salad with dressing on the side as your entree. This will be a good way to get some micros and volume into your meal without excess calories.
9) Go for fruit or sorbet for dessert as they will generally be the better and more nutrient dense options.