Table etiquette can be challenging. Every GM is different, and every group is different. Even different systems can encourage different sorts of behaviors from your players. And for me to tell you, "Here are my set of rules and they are absolutely correct for everyone” – well, that’s more than presumptuous, it's just unreasonable.
However, a few years ago I was running three games at once, so I had to write some rules down and post them on the wall behind me - partly so I could help establish a solid starting point for each group, and also because I used to sit in front of a mirror, and I needed something to obscure my notes and dice rolls, so the players couldn't see when I flip open to the "Beholder" page of the Monster Manual.
There are a few rules everyone already knew - mostly things like asking players to stay off their phones, and a zero tolerance policy for rape jokes - but we were at the point in my groups where I didn't need to post those as written rules. These, however, are the ones I started to hang behind me as we play:
If You're Uncomfortable, Say Something. I don't want anyone to feel unsafe or uncomfortable, or that their ideas are unwelcome. Ask for a pause so we can resolve any issues.
I'm lucky, in that this has actually never been an issue during a game. I've had to deal with a bit of drama outside of games, but I'm fortunate enough that I've never had to stop a game to deal with a player's behavior, or to address someone's concerns. That said, it's important to me that everyone I game with knows that my table is a safe space, and the easiest way to do that is to write it on the wall where everyone can see it.
Share the Spotlight. Don't talk over other players, and do give everyone room to role-play their own scenes. Additionally, if a character wants to have a private or personal moment, give them space to do so, and respect the scene.
This is one that did go onto the list after a few specific examples in groups. And this is one that takes a while to instill in people. Let's tackle the first half first - if one character has a one-on-one scene with a non-player character, it's just one player and the DM talking. For some players, this is the perfect opportunity for the other players to get up, stretch their legs, or get a snack. However, I much prefer that everyone else patiently sit during the scene - it gets pretty distracting if people are wandering away and having other side conversations during a game. Even if your character isn't in the scene, I'm of the belief that you should show the other players respect.
That's really what this rule comes down to in general - respect for the other people at the table. As we address the second half of the rule, this really comes into sharp relief. More than once, I've had players tell me they want to have a private moment that nobody else sees, and immediately other players try rolling perception checks to see if they notice it happening. The scenes weren't even necessarily that important - they were just quiet beats, a bit of character development that only one character is aware of. And not everyone needs to see that and know about it. When other people try to steal the quiet moment from that player, and force themselves into it, it can really dilute that moment.
This is something that I first picked up from actual-play shows (specifically Critical Role and the One Shot and Campaign podcasts), but to be fair, those players all have experience in improv. They're used to sharing the space, allowing other people to have their moment, and knowing what final line is the perfect time to end a scene. Chances are, your players won't have that same level of experience. But I promise, if you can convince your players to sit still while other players are talking, and if they can allow other players to have their own quiet moments (within reason, provided they aren't being prohibited from participating in the game as a whole), then you'll notice a difference in how your players interact with each other.
Keep Rules Debates Short and Sweet. When in doubt, settle on a house rule and look up the real rule later. And remember, the DM has the final word - sometimes he may forget a rule, though, so friendly reminders are welcome!
Oh my gosh, this rule is so important. Again, I'm lucky, in that most of my rule debates are short and sweet. I was part of an actual play for five years, so I got used to just pausing and looking up rules as quickly as possible, because we didn't want to bore our guests but still wanted to get the rule right.
And, folks, I know I just said it, but it demands repeating: your DM is not infallible. Your DM will forget rules. And friendly reminders are totally fine. What you don't want to do is challenge their rules. Just asking, "Wait, isn't this the rule?" or "Could this rule actually mean this?" - those are totally fine. But again, keep it short and sweet, and when the DM makes a ruling, respect it and move on. If you have any issues with how the DM resolves a rules discussion, take them aside after the game and let them know. Not every debate needs to happen with the entire group. If you feel the rules are being abused by the DM, or that a ruling made by the DM has limited the way you play, let them know about it politely and respectfully.
PCs Shouldn't Roll Persuasion Checks Against Each Other. Deception and insight checks should be used sparingly against other players, and be treated as significant. Persuasion checks against other PCs rob players of their agency. Dice should be used to resolve interactions with the world - role-playing resolves interactions with the party.
This was another one that had to be added to the list after some issues at the table, and I feel pretty strongly about it. I think it speaks for itself, but it's essentially something I've seen used to settle "arguments" in-game, and I've always noticed a frustration when a player is "persuaded" with dice rather than role-playing.
Be Nice and Have Fun!
It's literally the reason you're there. Have fun. Enjoy the game. Be excellent to each other, and party on, dudes.