Well, I've hosted a few LAN's at my house, usually about 8-10 people. I have also gone to many LANs that were 20+ people, one was about 30-40 people (
www.azlanenthusiasts.com).
For a LAN, follow Dexters suggestions, but also make sure you have some swag LAN games. Another note is to make sure you have lots to drink! Maybe not sodas (those work nicely, though it can cost a bit), but having some variety in liquid-intake is always good. Chips, and other snacks are usually eaten quickly, but having pizza, subs, or some other food with substance is a must.
The LAN's I've done run for about 8-10 hours on average, but it depends on how diehard your gaming pals are (and how much fun the games are
Oh, a couple games that have done REALLY well at my LANs:
- Carmageddon 2k
- Typing of the DEAD
- Serious Sam: The Second Encounter and Serious Sam (the original)
- Quake 3 Superheroes
- Team Fortress Classic
- Quake 1 (YES we still play it... so much better than q3 imho

- Science and Industry (Half Life mod)
- Age of Kings: The Conquerors
- Total Annihilation
People seem to prefer games that nobody is overpoweringly good at, so that they feel they have a chance. Plus, learning a new game and making mistakes with everyone else I find enjoyable
Oh, and one other thing... Probably the hardest thing to do is to keep everyone interested. Sure, there are the diehards who will play anything (me), but most people get bored after playing a game for an hour or so (sometimes less). Try to play games that everyone wants to play.
Goodluck! I hope your LAN is a lot of fun!!
Kamp