An important part of parenting teenagers is to teach them how to make decisions. Kids need to learn that whatever choices they make -- good or bad -- have consequences. Sit down and talk about some of the dangerous and long-term consequences that risky behaviors can have, including drug abuse, pregnancy, smoking, and drunk driving.
Know that no matter how well you prepare your kids, they're going to make some mistakes. The important thing is to show them how to learn from those mistakes.