Check out this upcoming event put on by Echo1 USA at the Desert Fox Fields MOUT Facility in Southern California:
If you're planning on attending this event, or a large event similar in nature, there are a few items I'd recommend stocking up on a few items beforehand.
Considering the event will probably allow the use of full-auto fire for all weapon types (not just Squad Weapons), you'll want anywhere from 5 to 10,000 BBs to take with you (depending on your style of play).
Extra rifle and/or pistol magazines would definitely be recommended so that you won't have to carry a bag of BBs in your tac vest (or even worse, have to go back to the parking lot) and refill all your mags during a game. That is lame. I've done it before and it sucks. I've missed out on some pretty legitimate firefights in the past because I did not bring enough mags with me out to the field. Don't be that guy (me).
You'll notice the rule about full seal goggles being required. If you rock the mesh goggles or shooting glasses, I would think about grabbing a pair of goggles for this one. It would be important to note that this event is not the first and certainly not the last event to require full seal goggles. Usually, you can expect that any major event is going to have this requirement, as it's likely mandated by the event organizer's insurance carrier or advised by their attorneys. So if you like attending larger Ops like this, it's a good idea to have a pair of goggles handy just in case.
How about food and water? I shouldn't have to sound like a mom here, but seriously. Bring food and water. Especially water. You can get large quantities of bottled good stuff from your local grocery store for cheap. So pick some up on the way out to the game. Don't be the guy that has to call a halt to the game while an actual medic (not the mil-sim kind) has to "casevac" you off the field for dehydration.
Might be a good idea to add an extra battery to your inventory if you only have one. You can chance it that someone will have one for you to borrow, but I've been to games where I didn't know a soul and didn't feel like asking some stranger to borrow his batt. That would be weird. Just get your own.
Last but not least and tied with the food and water element as the most important item to have with you at all times would be some good ol' fashion integrity.
Call your hits.
There is a special place in the darkest depths of the underworld for those that don't call their hits. You really put a damper on the day when you undermine the foundation of the game like that. Chances are, you'll be back in the game in a matter of minutes after you respawn/regen (depending on your regional dialect). So just call it, think about why you got hit and what you can do to minimize the chances of it happening again during your respawn time. You'll become a better player by learning from your mistakes.