Always a tough subject. My boys are older now, 19 and 17, so it's not as much of an issue for me anymore.
I'd have to say, the most important thing is discussion/conversation/education regarding media content. It's easy to police what they do at home but, when they're at school, or over at a friend's house, etc., you can't be there to guide them. But, if you instill in them values and an understanding about what's real and what's over-the-top for show, they'll have that to take with them and use as a reference for what they may be exposed to.
This interconnected, media at-a-glance world we live in today is much different than it used to be.
The other thing is just to accept that they WILL, at some point, see things they maybe shouldn't, but it's not the end of the world. My friends and I snuck in to see Alien when I was eight, and Excalibur when I was ten, among other things. I found my uncles' Playboy and Penthouse magazines around the same time. Being exposed to these sexual and/or violent stimuli didn't wreck my life or create problems for me or my family down the road.
Admittedly, though, the Internet of today is much more graphic than Playboy or Alien...