1) The LARP I was in for many years had a ritualistic Reading Of The Rules at the start of every event. The very first of these rules was a wise one: we should all be in this to have fun.
If people aren’t, something’s wrong. Change it. If I’m not having fun, something’s wrong; change that. If I need to take a break, then I should; it beats burnout.
2) Be true to (and aware of) yourself.
I run the game that I run, not the game someone else wants me to run. I’m ten times better off seeking players who enjoy my style than to compromise my style to please specific players. Beyond that, I should know what I can handle: how many players I can comfortably run, how frequently I have time to play, how long sessions should go, how much digression and socializing I want. Not knowing your own limitations ends in trouble. By the way? Articulate this to your players. I've been hugely wrongfooted twice; once, when I brought a serious, gritty assassin into a Top Secret game that turned out to be patterned after Get Smart!, and a Howard character into a game billed as based on Heinlein's Future History that turned out to be Monty Python meets Number of the Beast. In both cases I scarcely lasted out the first session. Like most players, there are styles I do and those I don't do, and you're a lot better off alerting me in advance.
3) Be prepared.
I not only run a sandbox, the PCs can choose to travel to any other city in the kingdom and there’s a book detailing the top ten people in local politics, how many temporal wizards there are, a paragraph or three of a hundred or more shops, what the major temples are, what the minor temples are ... It’s an appalling amount of work, but I can save my brain power to invent details my volumes of notes don’t cover, as well as not get caught short in contradictions ... hey, wasn’t the elderly priestess at St. Viria’s named Fidessa when we came through Seasteadholm in the spring? I thought you said the Sufontis Market was in the Zhantil District? And so on. However ...
4) ... don't overprepare.
The detail I want, as a player, is the detail I'm not only likely to encounter on my own, but detail which I reasonably think might pertain to the job at hand. I don't need to have an hour of session taken up by the GM droning on, a paragraph apiece, about every crew member on the merchant ship, from the head cleaners on up. How about spending that time working out the possible responses to what we do in reaction to your plot? I assure you I'd rather you had a handle on that than the hometowns, marital statuses and off-duty fashion details of all three bosun's mates.
5) Don’t ever, ever railroad.
It is not my job to tell the players what they’re doing. It’s their job to tell me what they’re doing. If they’re not interested in my plot, they’re not. If they make all the right guesses, then they have a walkover and I need to give them something else to do. Hey, how about a shopping expedition and a night on the town while I resign myself to more prep work for next time? In the meantime, what is my job is to have as many of the bases covered as is feasible. A clever party should be able to come up with a dozen ways to get past any problem. A clever GM should be able to foresee that they will and have a notion as to how to handle each choice.
6) Know your party.
I've heard from too many GMs who had the rug ripped out from under them by players reminding them that they had certain abilities the GM forgot to take into consideration. A prepared GM doesn’t forget these things. I keep copies of all character sheets, and I have a cheat sheet on a clipboard detailing Advantages, Disadvantages, stats, weapons of choice, defense rolls, reaction mods, Perception and Will checks and the like, for each character.
7) Don’t get bogged down.
If I can’t calculate the modifiers in the haggling session between Lady Sula and the goldsmith (the smith doesn’t give a damn for the aristocracy, Sula’s a babe, they’re finding each other’s accents a bit tough to follow) in an instant, then I should fudge it without hesitation, and if I can’t do that, I’m in the wrong business; there’s nothing more boring than watching the GM flipping through a stack of rulebooks ten times an hour. That aside, scenes should only take so long. NPC soliloquies should only take so long. Players should only get so long to meander or do their solo stuff. Adventurers and plot arcs should only take so long. Even an epic tale has its sell-by date. Brevity is the soul of wit. Keep the pace moving at all costs. (In combat, too. Combat rounds in the game I play are three seconds long. If the player -- who’s been cooling his heels for a couple of minutes anyway -- can’t decide what to do within ten seconds after I call on him, I skip him. You should too.)
8) Be a good actor and storyteller.
You play everyone else in the world. You set all the scenes. You handle much of the dialogue. If you can’t act and refuse to learn, you should be refereeing miniatures wargaming instead. Practice this. Use body language, posture, different voices and accents. If you don’t know how, learn.
9) This is a cooperative exercise.
Something you need to hammer into the players, if need be; however illogical, this is a consensus-driven game which needs to be handled consensually. A player who designs a character wildly at odds with the others, a player who wants to freelance all the time, a player who doesn’t want to get on board with the milieu or the setting, these are people who need to be told No. There are RPGs out there for rugged individualists who don’t want to act in lockstep with others; they call them MMORPGs and LARPs. There's also a role for GMs who can't bring themselves to say "No:" it's called "player."
10) Use no complexity in the game system you can’t readily handle, and avoid anything you don’t really need.
There are few things, short of drunk and disorderly players vomiting on the battlemat, more disruptive to the flow of a game than a lengthy rules debate. A lot of RPGs out there have “light” versions or a spate of optional rules that honest to God are “optional.” Don’t let this happy truth slip past you.
11) Know Your Stuff, or Don't Run Campaigns That Require You Do.
I'm an elitist ... more detail on this in another post. I think it's incumbent on GMs to learn as much as they can about their milieus, and play them as accurately and realistically as practical. I really don't want to see howling anachronisms, except in genres where it doesn't matter (30s pulp, for instance), or where the GM has an explanation in hand. (Yes, I recognize you might not give a damn about verisimilitude, but I warned you in the very first post about my philosophy.)
12) Believe in the Rule of Cool.
If a player does something outrageously cool in combat, let her pull it off. If a player comes up with a really cool idea, reward him. This will almost never go wrong.