The following 15 rules (warning: strong language) were written some years ago in Great Britain, and have been pretty much generally accepted on the British larp scene. Especially popular is rule 7 - widely known by its number and commonly considered to be the most imortant rule of all (and I agree). Even the biggest British larp forum has taken Rule7 as its name.
The rules have been originally created by the Drunken Monkeys and edited by Rick Wynne who added some extra stuff in the explanations to make them more understandable to international audience (it still contains some British larp lingo though), more work-safe and to throw in his two cents. (copy of the original wording is available here)
1. Don’t play a mighty warrior; play a warrior and be mighty.
Don’t label your character. As soon as you say that you are the best swordsman in the land someone will come along and kick your ass. Just get into the mindset of the person and role-play it out.
2. No one cares about your character background.
Really. Don’t tell me about your character and how great he is. It gets boring after the first 50 people or so have told you the same thing. The character is important to you, let that be enough.
3. You are not the Highlander.
There is an urban myth that after the first LotR movie came out no less than 9 characters called Legolas turned up to the Gathering. You are not the only one.
4. No one likes an elf
Are you playing an elf because you are genuinely interested in playing an alien mindset, or so you can be a human with pointy ears? Are you going to be yet another namby-pamby, wishy-washy, lowest-common-denominator, bleeding heart liberal over-used hackneyed fantasy concept? Your race doesn't make your character different from the real-life you, the way you act does.
5. You are not your vet picks. You are not your skill list. You are not your fucking lammies. You are not unique. You are the all singing all dancing shit of the system.
Your skills/ special abilities/ mingy items of ultimate doom do not define your character. Who he is, what he does, and what he wants are the things you should concentrate on. Most importantly do not try to be unique, fitting with the concept is better than breaking it. Min/maxing power gamer munchkins are role play killers.
6. Your character doesn't go clubbing.
Some folks think there is nothing sadder than seeing people in clubs wearing their larp kit. People walking around in-character like they have just stepped out of a Rocky Horror Picture Show is equally as bad. Kit or goth, don’t confuse the two is the saying. Not sure I completely agree but one thing is certain, pick your kit to match the IC setting!
7. Don’t take the piss.
You heard the man.
8. I do not want to find your character in a book.
Larp is littered with plagiarism. It’s not big and it’s not clever. Be inspired by books, fine, but at least change the damn names.
9. Drum-rolling is for marching bands.
Weapons, my friend, are pretty damn heavy. Act like it. I want to see huge, heroic, Conan-like sword swings! Tippy tappy flicky fighting makes it look like you are fighting with foam. That does not mean you need to move in slow motion!
10. Being hit hurts, bitch.
When someone hits you it hurts. A lot. Even if it doesn't kill your character you should be crying like a big girl. React to being hit, no matter what type of armor you are wearing!
11. I don’t give a damn what would really happen.
The “what would happen in real life” argument is the first resort of the loser. Repeat after me, “it’s fantasy, it’s just a game, this is not real life.” Its not re-enactment. Don't worry about authentic, but try to be coolthentic.
12. Yeah, but you didn't. Get over it.
Quit living in the past, man. Let it go!
13. What’s an “out-of-character”? Are you ill?
If you’re in an in-character area, and its during time in, then be in character. If you want to be out of character for whatever reason then go somewhere else. You are only spoiling the enjoyment of those around you. We are all props in each others' game.
14. They only killed your character. Get over it.
Invest in your character to get the most out of him. When he dies, though, just let it go. If anyone starts moping and bitching because their fictional character has ceased to exist then they’ll get a visit from the Sad Bastard Police, Muppet Division.
15. Your character is not you in fancy dress.
If you need this rule explaining to you then you also need your head checked. Leave the event and go see a doctor.