The more people you have at your LAN, the more important this one is. We had one party where a friend decided to bring his subwoofer. Eventually everyone was screaming for him to turn the damn thing off, and a few people were getting sick. This is also an important rule if you play any Real Time Strategy games. If the other players can hear what your units are saying, they get a nasty advantage.
Headphones are practically mandatory.
Bring as many hubs and cabling as they can, and make sure they are labeled. Nothing is more frustrating than bringing a computer to a LAN party and not being able to hook it up. Also, for tear down, labeled hubs and cabling makes things much easier. Bring all your software in them. Jewel cases take up way too much space.