If you're not at home (eg. running errands) and you find that you're hungry, find a grocery store and run in. You can get tons of healthy snack options in small portions for very little money! This lets you satisfy your hunger without resorting to the drive through!
Plan meals for a week at a time--and more than just dinner! Plan breakfast, lunch, dinner, and even snacks if you want to. This makes you less likely to cheat or eat junk. You can also "build in" times for eating things like birthday cake--eat light that day so you can enjoy the treat guilt-free!