Well, I have only tried pot and I've never been like puking drunk (and had a hangover twice).
What my mom did, was anytime anything about drugs or alcohol came up in the newspaper or tv, she DRILLED it into my brain that "kids who start drinking early become addicted more easily" and "your brain isn't fully developed as a teen and if you drink or do drugs you could permanently damage your brain or stunt your brains growth" also "the first time you try ecstasy you could die, it has happened" and "pot is a gateway drug" (but pot isn't actually a gateway drug, its probably more of a starter drug, cause I tried it and I'm not all over drugs) also that heroin is highly addictive...etc.
I have also in my college/adult life done work with addicts, and from what I can see that - no offense to addicts - but their lives suck. Their children are taken away, sometimes for good - they commit felonies (DUI) and have to deal with drug tests, parole officers, not getting a job - it's hard to not relapse and its hard to recover and its hard to make a living.
I would look up some serious facts (don't try to flat out scare them out of it, but with my mother being pretty insistent, I got the message) and put together a sort of outline for what you want to say. And repeat this message whenever it comes up. You'll kid will act annoyed, but their listening. Those anti-drug ads are true, you kids will listen even when they look like their not listening.
Oh yeah and don't be too slack, cause the kids that stay out too late for their age, have no strict set of rules are the ones who go and drink, smoke, etc.