Trust us, we already survived one of these larps. We know what it takes (or at least what you'll really miss not having).
Safety goggles/glasses - You absolutely must have a pair. Must be worn when moving about outside of the designated safe-zones. This is to protect your eyes from airsoft ammunition and miscellaneous debris. Do not use net-style eye protection unless you wear regular glasses under the net. A net will not protect you from dust, sparks and debris from pyrotechnical effects. If you are unsure about what this means, come chat with us.
Flashlight(s) - Bring 12 if you have them. Don't stop until you're reasonably sure that you'll be visible from space. Bring back-up batteries. It gets very dark at night and there are plenty of poorly lit locations you may want to explore, even in day time.
Warm sleeping things - Night time most likely means "holy mother of nuclear physics, it's cold out here" time. Especially when you're exhausted, unwashed and sitting still. Bring things that will keep you warm. Bring things that will insulate or elevate you from the ground chill.
Painkillers and simple band-aids - Headaches and scratches will happen, and when they do you'll want to be prepared
Food - To eat for the whole weekend, things to keep you on you on your feet. Also delicious snacks.
Canteen/water bottle/skin - Perfectly drinkable water will be available, but you'll want something to put it in.
Gloves - Helps your hands stay protected from rusty, dirty, dusty things and helps you stay warm at night. They also have neat protective capabilities against airsoft BBs. Bring an extra pair.
Bring the above and you should be good. We also have some things that you won't need to live but are super-awesome to have so bring them anyway:
A hand-held personal radio - this will enable you to listen to the radio broad-casts! And you'll want to because they are awesome. As a side-effect, you'll also be available to communicate with friends (and enemies) across considerable distances
A mug or cup or other concave vessel fit to carry liquids found in a bar - there will be a bar. It's more likely to serve you if bring your own glassA good hat and scarf - airsoft ammunition can smart quite a bit if it hits your bare skin. If you think you'll put yourself in a position to get shot at, you'll want to protect your precious face/neck.