According to Carole Maggio, creator of Facercise -- exercises to tone your face -- heredity plays a large part in whether or not you have a double chin. One of the things that can emphasize a double chin is smiling, but you don't have to resign yourself to a life of solemn expressions. Fortunately, if you practice thinking before you smile, you can implement a couple of tricks that will help you avoid making a double chin.
Press your tongue to the roof of your mouth to keep your chin from dropping down and forming a double chin.
Extend your neck forward -- not up -- like a turtle to stretch out the area under your chin before smiling.
Practice the techniques in front of the mirror to see the effects and make any needed adjustments. For instance, you may be sticking your neck so far forward that it looks unnatural.