In this post I'm not talking about being the person at the table that has memorized all the rules or maximized the powers of their character.
No, in this post I'm touching briefly on how to be a good guest.
Rule One - The character you are roleplaying is not a convenient excuse for you to say mean things or set up the character of a player you don't happen to like. Sure, you should roleplay your character but remember that telling another player, even in character, that their ideas are stupid or that their character is worthless is just a very thinly, and poorly veiled verbal attack against another person who is a guest of the DM.
If you dislike the other players that much and you think you are such hot shit, do everyone a favor and find another group. All you are doing is making an ass out of yourself and proving to everyone what a complete dick you are.
Rule Two - RSVP. If you receive an invite for a game, even if its an invite back for the next installment of a current campaign take the ten seconds out of your day to email or text the DM back and confirm that you are going to make it. Hand in hand with this rule is giving the DM as much notice as possible that you can't make it. Avoid waiting until the day of the game or a few hours before game time to tell your DM that you are not going to be there. The DM has poured hours of their life into organizing a fun social function. They think enough of you as a friend to include you on the short list of people they are inviting to this event. Probably they plan to drop some personal money on books, props or food and drinks. Sure, life happens, but let the poor DM know if you have to bail on a game so they can plan for your absence.
If you decide the game isn't really for you, just tell the DM. Be up front. Don't drag everything out, agreeing to show up for the next game and then cancelling at the last minute over and over. That sort of thing is just a dick move. The game will survive without you but you'll make the DM's life a lot easier if he or she knows to start looking for a player who actually wants to be there.
Rule Three - Leave politics and other sensitive topics at the door. The people gathered at the game are there to relax and laugh and have a fun time. They don't need a player playing quiz master about their personal religious or political beliefs and then scoffing at their beliefs.
Rule Four - Game night is not the night for you to hit on another player you happen to find attractive. Probably the other player is already in a relationship and even if they are not they didn't come to game night to get your phone number. The other players came to game night to relax. They want to be themselves and not have to be on guard against some other person making suggestive comments or flirting with them. Give people their space. Respect them.
Rule Five - Help the DM out. Offer to contribute to the snacks or drinks on game night. Don't just blast out the door ten minutes after the game is over. Stick around for fifteen minutes and thank the DM and everyone else for hanging out. Help clean the place up.
Rule Six - Don't cheat. If your a player you should trust that your DM is going to try to help you have a good time. If you roll a seven then just say you rolled a seven. Don't try to bullshit your roll and claim it was a seventeen. You would be surprised how quickly DM's and other players will spot your cheating behavior and all it will do is make them think about what an insecure, poor sport you are. If you are cheating on rolls and your DM is anything like me, you will probably find yourself no longer invited back to games.