I agree with Trillian. Bring it up in conversation (hard to do I know ... "can you pass me the carrots John and yeah what do you think about Penthouse these days?").

Perhaps wait until something porn-related happens in the news or if something's going to be on Today Tonight or ACA about online porn and how easy it is for kids to access it.

And I agree with Schmickers - emphasise that the bodies and the sex you see in porn is not real life.