Get to know your teen.
I was astonished when I was talking to a 13 or 14 year old girl, and she was pretty quiet and really sweet, and her favorite star was Paris Hilton.  And, I was like, "when can you even see her, she's not in the media that much".  "Oh, I've always wanted to be Paris Hilton for like three years."  And, you're thinking, "Really?"  But that's what was important to her.  It may not have been who I saw sitting in here, but it's who she really wants to be.
And, the other thing I think is a great tool for parents-- even if you don't watch it with your teen-- watch MTV.  MTV is not about music videos anymore.  It's all TV shows now.  It's "The Hills", it's "My Sweet 16 Party".  Watch that!  Those are the dreams of 16 year olds.  It's a bit terrifying, but if your teen's watching that, why are they watching it?  And usually teens will say, "I hate that show. I don't know ever watch that show."  Say, "It's okay, let's just see what other teens are doing".  There's a TV show called "Sixteen and Pregnant".  It's popular enough that it's on MTV.  What does that say about what your child is seeing, and what they think is okay or not okay.  And, you can watch it and have your own personal education.  It's a pretty big deal.