1. Arrive on Time
...So everyone can start together, and you don't miss out on stuff if we start without you. On-time arrival communicates mutual respect for all the other players gathered. Things come up, schedules get busy, emergencies happen - but communicate these things so the GM can plan accordingly.
2. Think about your turn before it happens/get organized
This is a life lesson. Get thyself organized; everything will take less time, you'll be less stressed, and EVERYONE will appreciate you more. ;)
If you're a monk with a million attacks...maybe roll your damage and attacks together. Then, if the attack hits, you already have your damage. If it doesn't, discard the rolls. Easy-peasy. Set your dice out that you're going to roll ahead of time. If you're hasted (ugh), maybe even write down your own order of operations to keep yourself focused on your tasks, then check them off one by one.
If you're a caster, READ YOUR SPELL. Know what it does, understand its range, casting time...all those frustrating details; but ESPECIALLY have ready the Save DC and WHAT exactly the DM is supposed to roll, if there even is a save. Bonus: casters who narrate what they do to cast the spell adds great depth to their character and the visual art of it all, plus demonstrates an understanding (maybe) of how the spell works, so try it out!
(In 5E) The Attack Action, even if you have Extra Attack, consumes your action, so unless you are Hasted, you cannot also cast a spell, or interact with an object, or grapple a foe, or do anything big after that. Your movement moves you across the board...that's it. A Bonus Action is specific - if you do not have a feature that would grant you a bonus action, assume that you do not have one to spend. An understanding of this speeds up play dramatically - and actually allows a lot more to be done each encounter, as more player turns will occur - this way no player feels that they need to CRAM their turn with stuff to do because they won't get another chance later.
As a DM, I've made a few house rules for player ease: 1) Potions take a Bonus Action to consume yourself, but an Action to give to others; this way, there's still a cost to healing, but it doesn't consume a full turn. 2) Sometimes, as context or creativity might allow, I have expanded the use of the Bonus Action - this is something I will try to do less of, as individuals have become confused by it/try to abuse it. 3) As per the rules, a Bonus Action spell does NOT allow another leveled spell as an action; me and my fellow DMs rule that one can cast a Bonus Action spell AND another spell of 2nd level or below (but the core rules state "cantrips only" for this).
There ARE class features/feats/items that mess with these basic rules, of course, but everyone is held to that initial standard. Knowledge of what rules you can bend, and what you can break, will speed up turn time and allow more opportunities for everyone. PLUS, it helps the DM avoid unnecessary rules lawyering/arguing/etc. that can drag down play.
3. Share the Spotlight
This is a group game. Don't hog the spotlight.
This requires some personal and social awareness. There can be a lot going on, but no one wants to feel forgotten. It gets harder with bigger parties, but if everyone stays aware of their own spotlight, no one should feel slighted.
It goes a long way, especially if you think that what you're about to do is going to take some time, to INVITE another party member, maybe one that isn't used to playing much yet, along for the ride. This makes them feel wanted (yay), gives them a joint opportunity to shine, AND forces the initial player to interact with their party in character. Maybe the whole group follows - sweet! I'd rather an ensemble with everyone taking part than a looooong solo with everyone else waiting.
4. Listen to the Game Master
If the Game Master is talking...listen. Your side conversation can wait, that random thing you want to say that you just thought of...can wait. This isn't me trying to squash anything; quite the opposite really. It's to set up a level of respect. If the GM listens to you as you describe something, then listen to her as she describes something.
Your GM works hard for you; they have a tough job, and they need to be given the space to help paint the picture in front of you without being interrupted. Some great emotional and effective moments can be hindered by a player making a joke in the middle. Don't spoil the fantasy of a detailed resurrection ritual just because you thought of a thing - save it for later.
Plus, this ensures that everyone comes along for the ride in the fantasy, at least for a little bit. Details are all shared; collective in the theater of the mind. It grants the DM space to craft a great picture. Seriously. Shush.
5. Rise to the Challenge
...and don't whine about its difficulty.
As player-characters get stronger, they will undoubtedly face off against more and more powerful entities. Wizards are SCARY at high levels, and some dragons are ALSO wizards, so... Mind Flayers are terrifying; there are some creatures that will just bring your hit points to 0 as an ability; Legendary Actions keep you humble; Intellect Devourers, though weak, can still decimate your party. ALSO, heists are hard. Planning is hard. Maybe you're not a sneaky person, maybe you're not a planner, maybe you just want to set everything on fire and call it a day.
We can use these moments in a fictional game to help ourselves become more complete people. Rise to the challenge to improve your planning, your execution, your self-control, your spell usage, your creativity - don't whine about it, TRY. Because anyone can roll a NATURAL 20. :)
Have an awesome day.
See you at the table.
Professional Game Master musician, music teacher, game designer, and aspiring fiction author.