The scouts taught you to expect the unexpected, but when dealing with a LAN Parte` you have to expect more than that. Expect the unexpected, the unlikely, and most of all what seems to be the impossible!
It is a well-known fact that it is physically impossible to know the number of people who attend a LAN Parte` until all of the guests have departed, and this bears no reflection of the number of people who knew about it, or even have the required equipment. The attendance is also inversely proportional to the size of the house and the available space for the LAN. (This of course means that as more people arrive and the space gradually decreases more and more people start to arrive.)
A power extension lead to reach the neighbours is a good thing (for when you run out of fuses and the power company refuses to turn your power back on because they say you are "dangerously abusing the grid" by using a 48 ampere fuse)... However a three-phase coupling kit and a tall ladder is definitely a better thing :)
An ample buffer of food supplies is convenient to have, but an inexhaustible 24/7 food outlet is essential! That corner store might be near by, but when you've worked down to the dusty box of stale Doritos the store-owner found in the back store (which he claims is perfectly good, despite the 1998 use-by date) you'll be wanting an auxiliary source!
It's generally a good idea to have a convenient apology memo for your neighbours...something in the ballpark of "sorry that you had to wake up to repeated shouts of swearing at 4am this morning" You never know, it might stop them calling the cops...
Spare 'stuff' is advisable... namely NICs, UTP Cables, HUBs, Hard Drives... and anything else you can be bothered to gather, cause god knows you won't be bothered looking for it when the LAN gets going and you could be sensibly using your time for fragging fun!
Make sure you have ample accommodation for the LAN go-ers, especially when they claim that they have no intention of staying the night. Beds should be in the vicinity so that after 96 hours of sleep deprivation people can conveniently collapse onto the mattress with no more than 3 steps. Alternatively the floor near their computer will do nicely, and teach them to evaluate the chance of staying over more accurately next time!
Most of all remember to have fun for the duration of the party! You can worry about repairing your now semi-destroyed house later.