Besides not wearing it at all, the biggest mistake people make with sunscreen is not using enough. When it comes to applying sunscreen, more is better.
As a general rule, use an ounce—about a handful—to cover your entire body. Put on enough that it takes a full minute to rub in.
Also, make sure you go through this application routine 15 to 20 minutes before going out in the sun—not when you're already outside. Then, reapply it every two hours and after swimming or strenuous exercise.