What is metagaming? It is the process by which a player, takes information they know casually, and applies it to their character.
For example, out-of-character, Allan and Rob are hanging out playing Earthworm Jim and talking about the LARP. Allan launches the cow and mentions something about how he'd love to have that cow land on Rob's character. Rob asks what Allan means, and Allan reveals that Rob's character is the best-friend's roommates cousins sister's uncle of his grandsire who was an Infernal diablerist from Pluto. And that the voices in his head have been telling Allan's character to pants Rob's character in the middle of Elysium.
So Allan has told Rob, outside of the game, all of this. Allan's character has not revealed his Infernal orders to Rob's character. However, Rob does not want to have his trousers hit the floor in the middle of game. So, during the next game, Rob's PC keeps avoiding Allan's PC. This is an example of metagaming.
Another example: Brett and Jeremy are hanging out trying to beat Allan's score at Earthworm Jim, when Allan bursts into the room and sprays everyone with Jell-O. Not even good Jell-O, but that lime-green nasty high school cafeteria Jell-O. Allan laughs maniacally, tosses the cape (really, just a bed sheet safety pinned to his collar) behind him, and dashes off. As Jeremy and Brett sit there, the ill-flavored gelatin slowly hardening all over them, they decide that Allan must be put into his place. Together they decide the easiest way to do this is to whack Allan's PC in the LARP. Their characters have no reason to want Allan's character dead, but they "fudge" it so they can get away with it. This is metagaming.
Metagaming is bad. Metagaming leads to more arguments, accusations, and errors than any other thing in the LARP. Intentionally metagaming will earn anyone a strike, even an ST. Guess what happens after three strikes?
There are a few ways to avoid metagaming:
"The first rule of Vamp Club is: You do not talk about Vamp Club!" The easiest way to prevent metagaming against you is not to talk about your character OOC. Don't tell anyone that you're an Anarch infiltrator, or a member of a bloodline people fear and despise. Do not tell anyone that you're planning to seize Praxis over the Domain. Don't tell anyone what your selective feeding is. Metagaming guilt very rarely ever rests on solely one party. And that makes it hard for STs to adjudicate. If you don't want something getting out, don't say anything. Simple.
"The second rule of Vamp Club is: You do not talk about Vamp Club!" So, Bob just told me that he's planning on wiping out his whole clan. There are a couple of ways I can take this: 1) He wants me to help. 2) He's bragging about how billy-bad-assed he is. 3) He's sounding me out, to see to whom I report this information. 4) He's judging my reaction, to see what I'll do about it, if anything. Vampires are inherently duplicitous creatures. I'd rather not have the players become so conniving, but it has been known to happen. If somebody does decide to let you on to some information, ask yourself: Why? What does s/he have to gain? Whatever the answer is to that question, try to be the bigger player, and say something like "I don't want to know." Ideally, before they even tell you that they are planning to wipe out their clan.
Keep your sheets to yourself: Don't flaunt your character sheet around, it holds weaknesses as well as strengths.
If you do decide to tell someone something about your character, do it with the strict understanding that it's OOC.
Maturity rules: This is a hobby, it is not a lifestyle. Make sure the game maintains its proper priority in your life. If you have to work, do not skip it for LARP. If you have a date, do not skip it for LARP. If you have a paper due, do not skip it to LARP. If you have to clean your room, skip it to LARP.
Remember that it's just a game. This really should have gone first, as it's the single most important rule. Do NOT jimmy the lock to your IC rival's dorm room to peruse his e-mail and see whom he's plotting with. If you want to find things out, do it in game or over a downtime. DO NOT interrogate, B&E, torture, snoop, blackmail, etc.