1. THOU SHALT BE RULES-PREPARED: Always be sure that you have ample time to read the different rules and understand just what major discrepancies in the new rules will have the most impact upon the character you will be playing.
2. THOU SHALT BE PROACTIVE WITH THE GM: Be sure to take at least a few minutes time to spend with the Game Master before the game so that you can have all of your questions concerning rules differences answered at the source.
3. THOU SHALT BE FLEXIBLE: Be sure to give the GM ample time to read over your character sheet so he can tell you what changes, if any, must be made to your character in order to be compatible with the GMs style of game play.
4. THOU SHALT BE TRANSPARENT: Let the GM also peruse your magical items, spells and other special abilities/powers so that he will not be surprised in the middle of a melee with an unfamiliar device or such. Most GMs react to such surprises quite simply, by nullifying whatever it was you pulled out of your bag of special tricks.
5. THOU SHALT INTERVIEW THE EXISTING PLAYERS: Try to find time to converse with the other people who have played/are playing in the GMs game so you can get a feel for them and just what the GM's game, world and style of play is like. If it appears to you, after such discussions, that this is the kind 0f game you might have a problem with, then for heaven's sake reconsider your participation.
6. THOU SHALT KNOW THY CHARACTER'S ABILITIES: Be sure that you are totally familiar with everything about your character before the game begins. In the middle of a raging battle is not the time to dig out a rule book to determine just what your character's spell effect is.
7. THOU SHALT BRING THY GAMING STUFF: Bring all the equipment that is necessary for play in the game. Different games occasionally require differently numbered dice and so on. If the GM wants all characters to be represented by a suitable miniature figure, have what is needed and do not borrow from the other players or GM.
8. THOU SHALT NOT BE DISRUPTIVE: Ask questions during the game but choose the appropriate time so as not to disrupt the play, if at all possible. Better yet, ask a player first, and only if he cannot help you, the GM.
9. THOU SHALT RESPECT THE FLOW OF THE GAME: If the game is touted as "many" hours in probable duration, come prepared with a snack that can be eaten where you sit. Don't expect to be allowed to saunter away to eat your dinner when-ever you feel like it.
10. THOU SHALT SHARE WITH THY COMRADES: It never hurts to bring a snack to share with your new-found friends. Friendliness and a willingness to share is an utter necessity if you are to enjoy the new experience to the fullest.
11. THOU SHALT BE RESPECTFUL: Do not be critical of rules or GM decisions in the new game. That's one way to cut your character's survival chances to nearly zero. Be open and receptive to new ideas, even if you think they stink. Just keep those thoughts to yourself, during the game. Afterwards, you must decide whether the new game is one in which you want to continue playing. If it is, then politely discover, after the game, why a certain situation was adjudicated the way it was. Who knows, your polite inquiry might get the GM to reconsider the ruling in question. It does happen ... if you are polite and reasoned about it.
12. THOU SHALT BE A TEAM PLAYER: Don't denigrate the situations or monsters you may encounter by comparing them to tougher or better or more unique situations in your games. No GM likes to hear people denigrating him. Besides, the other players will probably lynch you on the spot because they know that such remarks will cause most GMs to redouble the lethality of their play from that point forward.
13. THOU SHALT BE HUMBLE: Don't try to be a know-it-all or act like there is no situation or monster than you haven't seen, can't handle and so on.