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.
These things are right up there with hubs and cabling as far as importance goes. Somebody will probably forget theirs anyway, so try to have a few extra around. Also, know how the circuits go in the room. Know which outlets go to which fuse, and plan accordingly. If you get enough computers in one room, blowing a fuse can be a real possibility.