Been to two, hosted at a friend's house, so here's my 2 cents
1) Food. Drinks. Lots. Gamer's get hungry/thirsty fast, and having a drink or a snack near will keep 'em playing and keep 'em happy!
2) Make sure either you or someone helping you out knows how to set up LAN networks. Sometimes computers give you grief.
3) If we're talking about a general LAN, it would be a VERY good idea to either require people virus scan their computer's before hand, or distribute some free scanning software before they can get on the network. A virus would be a bad way to bring down a LAN.
4) People can bring their own games, but make sure you promote a few games in particular. You can get people who already have those games coming then, and it'll make for more interesting LANing.