Kim Waggoner, LICSW, says, "Your teenager is still neurologically developing. Your teen's brain has not yet reached the level at which he or she is always able to make the best decisions. Sometimes, it is truly difficult for him or her to judge what is risky. For this reason, parents need to be very clear about their expectations and what is not acceptable behavior."
Though it may seem like some expectations should be obvious, it is important to continue to remind your teenager about them (for example, no smoking, avoid alcohol, don't get in a car with someone who has been drinking, do not be late for curfew, always tell me where you are going and who you are with, etc.)
When faced with a difficult choice, you want your teenagers to hear your voice in their heads guiding them in the right direction. Ultimately, all you can do is equip your teenager with the tools to make the correct choices. You will not always be there to catch them and this is one of the harshest realities of being a parent. But you can take comfort that you know your child best and you know if you have given them the arsenal to make good choices (at least, most of the time).