When I was hosting large parties I had everyone fill out a checklist of what they brought in and then it was checked by staff at the desk to verify it before they were allowed to setup. This list was also checked before they were allowed to leave as well. Combine that with placing staff members at the end of every row, or scattered evenly among seats alot of the would-be thieves chicken out. That was the point, to try and prevent the petty thieves because honestly at a LAN thats all you can do. People who are going there with thoughts of stealing stuff will, those who look at something while there and say "hmm can I take that" will probably not.

One thing I also recommend is bumping admission fees up by 2$ per person. They wont notice the difference but it helps to start your "replacement fund" and over a few LAN's you should have a nice chunk of change to be able to replace anything that gets stolen.