It is important to begin well before a child is in their teens to address this issue. Explain candidly and in terms children will grasp at as early an age as you can about abusing drugs. Point out the positive effects of drugs that are given by a doctor and then address the negatives. Give them as much information as they can grasp at the time.
Try to speak loudly by your actions. If you abuse these substances chances are your children will. Thank God my daughter did not know that I was abusing prescription drugs. She never abused drugs of any kind and never smoked. She had some relatives that came close to death from abuse and that was a powerful lesson for her. .She also was friends with kids who did not use drugs, thank you again, God, for that.
Ideas: know what your children are doing, who they are with, and where they are at all times. Watch for tell tale signs of drug abuse including lack of interest in an old favorite hobby or pastime, weight loss in some cases (depending on the drug), smell of alcohol and tobacco, as well as a host of others.
Advice: Don't like to call it advice...suggestions....be open and ready to hear whatever your teen wants to hear and begin this well before they are teens. they need to know that no matter what you will be the wind beneath their wings to catch them if they are about to fall. That does not mean you should not be firm and have a no drugs policy. It only means that you will be prepared to listen nonjudmentally to whatever they need to share with you.
Kids are street smart today, much more so than even the kids of the early nineties. The world is at their fingertips so they know whatever you may tell them. they also can locate the things you won't tell them like the rush from a drug that they will read about on line.
The way you can counter that influence is to know what is going in on their lives, provide your support, your wisdom, and assurance that you will be there for them. If your child should stumble and abuse some drug, deal with it swiftly but try not to lose your mind. If you do you may cut off communication.
This is a very simplistic answer for a very complex issue but i hope I have helped some.