Broad "Listen Up Everybody!" advice on GMing often boils down to a list of things that'd be great if you had infinite time or a list of things that would be really dead-on if you were a totally different person.
Broad advice on playing often has the same problems plus it's for players so nobody reads it.
It's hard to give advice that applies to all the different problems people can have, but there is one thing that I think applies across the board to players and GMs in pretty much all RPGs and all playstyles and all personality types and that even experienced players and GMs can always push themselves to do better in every game (myself 100% included): support the reality of the other peoples' inventions.
What that means is: when someone (within whatever parameters the game says is legit) establishes they're doing something or something is there or something is like that--constantly casually acknowledge it in how you talk. It's basically a lot like the old improv rule: "Say 'Yes, and...'" but it doesn't ask for as much. You don't have to top them or even build on it--just mention what's there, whatever it is.
If the GM incidentally says the room is lit by magical fuchsia flames--say how weird everyone looks in the pink light. Mention looking down at the veins in your palms to see how they look. Or ask if it gives your pink warpig a stealth bonus. Not necessarily to get a bonus or find anything, just to say "Hey, that little detail--I heard that".
The druid says she has a scarf made from an ermine that (she hastily adds) died of natural causes, have your thief make a point of being like "How's that sound, dead ferret?" before nailing down the heist plans.
Do it in little ways: if you're playing in a pick-up or con game, try to use the other characters' names. Try to remember what they are good at and ask them to help. "Can you hold them off with your pike, Arnie?"
The only consistent difference between hanging with your buds and playing one of these games--the only reason we chose to do the second thing instead of just the first--is whatever it is we get out of a shared imaginative space. Simply reminding them "Oh you imagined that? I am now imagining it too" deepens what we get out of that, no matter which direction we then want to use that space to go.
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