I apologize that these have a strong fantasy flavor to them. I initially wrote all of this in a series of emails as advice to fellow GMs who run D&D or OSR games. I've rewritten it, tried to polish it up, and put it into a concrete list of rules that I can quote later primarily for my own edification and use. I've been GMing wrong for a long time, and I think I'm finally beginning to understand how a good GM operates.
#1: The rules, and the dice, are to be ignored whenever it is necessary.
When Joseph Goodman wrote "Let the rules bend to you not the other way around" he encapsulated a key philosophy in playing a tabletop RPG without actually giving it lip service. We're all here to have fun. If the rules get in the way of that fun, then they deserve to be ignored. Rolling for everything is bad too, because sometimes a character should just succeed. Every time somebody wants to do something, ask yourself "Would failure here be fun for them?" and if the answer is "No." then skip the roll. Climbing a tree to get a better view of the forest? Navigating the southern coastline? Studying to find a vital clue? Or even, examining the mountain range to find the best place to camp for the night and avoid an ambush? Ignore the dice and just get on with the game.
#2: No adventure survives contact with the players.
An oldie but a goodie. Don't construct an elaborate meta-plot meant to be uncovered over the course of a campaign, just chuck it all out the window. You might start with an initial plotline or story or threat, but if the players are smart enough to come up with something that seems like a reasonable (or better) explanation for everything you've already established then just change things up mid-game and use a twisted version of what they think up. No heavy lifting necessary, and the players will think they're brilliant for guessing something that turned out to be more or less "correct." Don't ever expect the players to follow your path or go for your bait either. Their ingenuity in the face of conflict should drive the story, not a list of bullet points you wrote five weeks before they all made characters.
#3: No lying, motherfucker!
You can have deceitful contacts, betraying employers, and allies with their own greater self interests. But the players deserve honesty. You can have secrets, but you can't have tricks. You can have traps, but their spider-sense should be tingling. You can have double-crosses, but they should know they couldn't trust that back-stabbing bastard! Always drop plenty of clues about the real intentions of your characters, you don't need to tell players outright what you're planning, but they should have plenty of opportunities to notice that something smells fishy.
#4: Don't make hard decisions when the players or the dice could do it for you.
You really want that noble to survive this assassination attempt. You don't want the players to fell your boss monster quite so quickly. You don't want the players following those gypsies to the west because what you have planned lies in the east. We've all felt these moments as GMs where what we have planned, or what we expect, is not happening the way we would like. Our story is being twisted, our awesome NPC is about to be killed, or our plot points are being ignored. Close your eyes, take a deep breath, now abandon those feelings. They're wrong. We're not sitting at this table to hear your story, or reach the foregone conclusion that you've decided between sessions, we're here to create a story together. In those moments you need to sit back and let the players' actions dictate the course of the story. Your prized NPC may die, but they have family that might seek revenge, don't they? Your dragon has just been killed and his treasure plundered, but nothing paints a target on somebody's back like spending a lot of money, right? If you abandon the story too much the action will begin to falter, and if the players aren't decisively plotting a course for themselves then you'll need to break out the dice and start cracking some skulls.
#5: Be a fan of the players' characters.
This comes from D. Vincent Baker word for word. Your job as a GM is not to be adversarial, your job is to make things fun and interesting. You can do this more effectively by giving them difficult decisions and adding consequences to their actions than you can by simply challenging them with dice rolls or taking away their stuff. Let the characters bask in glory and never deny them the success they've earned. When they've changed the landscape through their actions let those changes radiate out into the world. You're not playing from an "official campaign guide" so you can let your world adapt to the PCs in positive ways when they succeed and in worse ways when they fail, and even if you were using some "official campaign guide" that thing is just a guide not a bible. Put the fucking thing down once in a while! When the PCs do something important then every threat should be seeing them with fresh eyes, either appraising their new strengths and giving themselves pause or finding respect in their accomplishments and forgiving past grievances. Your world should be a dangerous place, but they don't need you constantly headhunting them too.
This is a personal preference more than a rule, and doesn't really need to be heeded
#6: Magic is unpredictably strange and deadly, let it be both without being crippling.
This is pretty self-explanatory, even if it is a longer rule, and a little confusing. Magic is the ultimate hammer for any nail, but it's a hammer that can only ever be swung wildly. If a player uses magic, they should get what they expect out of it, sometimes more than they want or expect. Fireballs should incinerate things, and set nearby objects aflame, and little contrails of sparks should shoot off of metal objects. IT'S A FUCKING FIREBALL! EXPLODE SOME SHIT WITH IT! Detect Magic should detect magic but maybe it's also detecting cheese around here, or detecting the last time somebody bathed. Invisibility seems to work but any nearby dogs end up curiously following them around. WARNING: If you're playing some version of D&D, magic is a scientific formula that players will rely upon with the precision of a calculator. Just tell them they're in a wild magic zone where weird things happen or some bullshit explanation like that. You might end up running an entire adventure where the PCs just try to find out where all of the wild magic zones begin and end, and man, if that happens, you should look at that as an opportunity to constantly fuck with them!