My 4th grader came home with a CD today. It's part of a Utah State drug-free curriculum. We've listened to the CD a couple times now, and it's sparked some great conversation between my daughter and I.
Anyway, there's this song called "Hot to Not." My kids LOVE it, and, of course, the message is great.
You can go here to listen to a sample of the song, read the lyrics, and look at the other materials.
There is tons of information out there about how to talk to your kids about drugs, alcohol, smoking, et al, but this is one that I think is particularly helpful.
Whatever you do, though, get a conversation going, and don't make it a one-time sit-down-and-talk thing, either. One statistic says that kids who say they benefited from drug discussions with their parents are 50-percent less likely to use drugs. You can read an article about it here.