My father not only told me that drugs were wrong, but he went into specific detail about WHY they were wrong, the damage they can do to your body, how stupid you look when you're on them, how your judgment is altered because of them, and the dangers that can go from there. He told me about overdosing, killing brain cells, addiction (and how painful detox is). He told me that by the time I realized I had a problem, it would be too late for me to do anything about it without help. He told me that lots of car crashes are caused by drugs and alcohol.
He taught me WHY I should say no and made sure that, by the time he was talking to me about it, I would never consider ever saying yes. The very idea of drugs and alcohol was stupid to me even when I was younger, and when I became a teenager and saw it all around me, I was never tempted. Not once. A lot of that probably has to do with my personality, of course, but a huge part of that is because my father took the time to go beyond the "don't do drugs" speech. He appealed to my sense of logic and did not stop until he knew I understood 100%.
It's because of him that I have never once tried a cigarette, weed, pills, powders, and needles. I was a virgin until I was 18 years old - literally the only virgin I knew. I did drink once, but that was overnight at a friend's house with just us two girls. If there had been more than the two of us there, I would have refused then too, simply because I wouldn't have trusted myself (or others).
The key to preventing your child's experimentation with drugs is to destroy the "cool" image around it. Let them know that, not only is it not cool, the person on drugs doesn't realize how stupid they look. They don't know (or care) the damage they're doing to their body. They're putting other people at risk when they get behind the wheel of a car. It's very easy for drugs to take over your life, and being a junkie is not attractive. Show them pictures of a pair of lungs from a smoker. Ask them if having a white stick in their mouth to look "cool" is worth dying of cancer.
And pray that your child is not a follower. Build up your child's confidence so that when his friends are pressuring him and making fun of him for saying no, it won't affect him. Hope that he has the strength to not give in to that and is able to shrug and say "if you want to do it, go ahead. No thanks."