Rule #1 is Know your ingredients
Find the nutritional information either in the restaurant itself or on the website.
Rule #2 Beware of the bread (or batter)
Avoid breaded or battered options in chicken and fish. If you chose a hamburger or hotdog remove the bun.
Rule #3 Forget Fried
Opt instead for something that has been steamed grilled or baked rather then fried.
Rule #4 Skip the French Fries
Or any potato for that matter. Chose a salad instead.
Rule #5 Simplfy the Salad-
Take off those croutons/tortilla chips/fried noodles. Also watch your dressing choice. (I've seen Light or Fat Free Ranch recommended most often. Raspberry vinaigrette and Italian have also been suggested on multiple sites)
Rule #6 Sauce on the side
Do not go overboard on sauces. Less is more. Ok less is less, but its healthier. :)
Fast food guide
Everyday Health Article on dining out.