Well after attending and organising lan parties here is the MUST list.
Decent switch + Backup switch
Spare power boards
Good quality network cables, make sure there are no old / kinked or damaged cables, they will spoil the day.
If you are using your internet connection make sure it is set up properly, consider a firewall especially for larger lan parties and try to restrict ports to the internet to prevent random torrenting etc.
Once you have a game list, have a couple of big usb sticks or external drives loaded wiith all the latest patches and updates for the games in question.
Arrange the tables for both maximum seating capacity while carefully routing power and network connections to avoid trip hazards.
the conference style setup is ideal with the switch at one end.
Some non-marking tape to cover wires, lessens trip hazard.
as for refreshments, it totally depends on the type of party, get some menus for local delivering takeout places, so guests can order deliveries if required.
Make a list of rules, mostly for the enjoyment and safety of the party.
Patrons are responsible for their own equipment, no liability accepted etc etc.
It is patrons own responsibility to turn up with updated software, while your patch drives may help, you are not there to just do tech support for a poorly set up system.
Check local laws as pertaining to prizes.
Make a list of tournaments and rules.
Possibly create a document agreeing to the rules that patrons must sign in order to take part.
If monetary contributions are required, state the expenses they cover and work out a fixed amount in advance.