J.D.
I have teenagers and this is a tough one. You want them to have a sense of privacy but not lose touch with what's going on. We installed a program on the computer that let's us koow what site's she's been on, and another that forwards us the emails. I know some people will say that's in an invasion of privacy, but would these same people drop their child downtown and drive away? It's not my child I don't trust, it's other people!
When my daughtetr's girlfriends come over the door can be shut. If boys are involved hanging out in the bedroom is not an option. It sents a really bad example of appropriate behavior. Unless you live in a house that resembles a 3 ring circus this should work Also, the best place to hear what's going on is the car. You'd be amazed what you can pick up if you just drive and listen.
Finally, don't assume that everything at 13 is so innocent. The world is a very different place then when we were young. If you start seeing changes in personality, school etc it may be a phase, but it also may be signs of larger problems. Be on the lookout! Good luck