Preparing for a lanparty can be time consuming so you better plan ahead.

Determine the amount of guests you want to receive.
A small lanparty up to 10 people can be hosted within your home network. Granted you have enough wires or even by wireless if you have a quality Wireless Router and friends with powerful laptops. Larger LANs might require you to buy or rent more professional equipment.

Location location location
If a LAN is small enough you might be able to have it in your living room, basement or garage. If you have more than 15 people you might want to consider getting a larger room. Ask your friends on Facebook/steam if anyone knows a large enough room to stay in for free! Help might come from unexpected angles.

Equipment
I don’t have much to say about the equipment you should get. Just make sure that when your network grows to over 50 people, you might want to consider a dedicated server to host your games on. Your game PC might be top of the line, but wouldn’t you like to spend that extra power on the game instead of hosting?

Power/safety
Make sure you don’t put more than 5-6 people on the same powergroup! Doing more could be a fire hazard and should be avoided. Run an extension cord from another room for extra power. (Make sure it’s on a different powergroup!)

Tables and chairs
Make sure there are enough tables and chairs for everyone! You might be able to rent some from a party rental store, or borrow tables and chairs from your friends for free!

Plan your games
It’s not really necessary to plan this ahead if you’re in a small group. However to keep things going in a larger group you want to create schedules for normal gaming or tournaments!

Plan a date
Not unimportant ;) There are a lot of websites that can help you pick a date with your friends!

Snacks!
Get some snacks,beer and energy drink! Planning ahead might save you and your friends a lot of money!

Patch up!
A few days before the LAN everybody should install their games and patch up! Limited internet access could be there, but don’t count on it! And you wouldn’t want to be the slacker that we’re all waiting for..

Have fun!
If you’re done gaming please remain in the game untill the following round! Ragequitting won’t make new friends :O