Eat sparingly on a special occasion. Sometimes you can split a dish with someone else. Especially so with dishes that are fried, breaded, scalloped, au gratin, or alfredo. Try to eat the same portions as you would at home. You don't have to order an entree.
Appetizers often come in low-calorie options.
If you must have dessert, order one with a fork for every member of your dinner party. Remember, if you have a bad food day, give yourself a break and get back on track tomorrow. This is a lifelong commitment. One day won't ruin it if you don't let it.