Double chins can be caused by one of three reasons: muscle slackness due to age, genetics and weight gain or being overweight.
Exercise can help, since a double chin is just fat around the neck area, so any fat reducing exercise can help. Also, gum chewing is known to work the muscles of the face and jaw, which can help reduce a double chin. Obviously, reducing your overall caloric intake would help with losing weight, which in turn helps with the problem of having a double chin.
A simple fix is standing up straight and tall; this gives the appearance of being taller and leaner, which is simply a visual effect only, but it will appear that your double chin is reduced.
Using makeup can help also, give the appearance of a slimmer jaw line and chin. Darker colors help things to recede, so using slightly darker makeup in that area can help give a slimmer look to your double chin. Clothing can be used as well, as a camouflage anything with an open neck or a very high neck can be effective. Another option is hairstyles, stay away from any cropped-to-chin level hairstyles. Wear your hear shorter or longer for the best camouflage.
Simple, free fixes, such as the above are helpful, but what if none of those give you the desired result? There is of course surgery, which should always be a last resort, as surgery is expensive and has risks and side effects with it.
Surgery for a double chin is called a lipectomy, and its a fairly simple procedure. A small cut will be made and fat will be removed and the muscles under the chin will be shortened. After the surgery, you may have a small bruise and a simple stitch or two, and you should be fully recovered after a week.
Be aware that lipectomy surgery is not covered by insurance, so it will be out of pocket cost to you. Also, some people have turkey-neck or loose and sagging skin after having the chin fat suctioned. In this case, you would also need a chin-tuck (or chin-lift) where the excess skin is pulled back to give a tight, youthful look.
No matter what route you chose to deal with your double chin, make sure it is one that works for you and your needs and desires. Start with simple methods and move on to more invasive methods if you are not happy with the results you achieve at first.