Partners For A Drug Free America have some excellent articles for parents wanting to learn about teens and drug prevention or rehabilitation. This is a great article about how to talk to your teens about drugs. Click HERE to read the article.
I would only add a couple of things: 1. Although I do agree that parents have a way bigger influence on teens than they may realize (precisely because of the picture above--teens always act like they aren't listening, but they are!), I am afraid that peers have an even greater influence. So, get to know your friends kids. It's important.
2. The article did not talk about a big issue that I see with teens today and that is that some of their parents are using drugs. Most marijuana and alcohol but sometimes pills. Parents often think their kids don't know, but they do. Just like they are really listening (secretly), they are also really watching. They can tell when their parents are buzzed or high. So if you are using and worried about your kids using, think twice about what you say and do. It's important.