It's been a while, but here are some new things I discovered/pilfered for my own use during my stints as GM. Also Dark, your OP is in dire need of an update.

[b]The most important rule for GM's[/b]
Your players are here to have fun. It is the GM's job to make sure they actually have fun. It doesn't matter if they want a flimsy story justification adventure where they crawl dungeons all day just to watch their numbers go up. Or if broody mcbrood wants to brood in your high adventure epic. They were accepted to be part of the story, make sure they remain accepted. Each player ought to be catered enough that everyone in the group has fun, not just one player above the others. Fun is to be distributed equally without distinction.
Remember, you are the fun polizei, and if you need to send jackbooted thugs to break down a players door and shove the fun down their throats so be it. All hail fun, the one thing each and every GM should strive for. Unfun will be sent to the camps of forgotten stuff.

[b]The GM is never wrong.[/b]
Everybody makes mistakes. While for most people it's difficult but usually necessary to admit those mistakes, GMs are never allowed to do so. That'd be like god showing up to a bunch of scientists and telling them whatever they're working on is pointless because it was just something he fucked up. "Sorry, lemme fix that, carry on." Therefore, never admit to being wrong. This does not mean you can force your players to do whatever you as GM want, because otherwise you are forgetting rule zero for GM's (see above). Instead, BE CONSISTENT. When players notice something you forgot, be all mysterious and applaud them for catching a subtle hint. Then out of their view plan around whatever it is that went wrong/you forgot/got mixed up etc.

Examples
GM: The dwarven fortress’ mighty gates suddenly appear before you when the sandstorm lulls for a moment-
Player: Waitaminute these were nomadic dwarfs!
GM: It’s a walking fortress of course.

Or alternatively:

Player: Hey how on earth is that mech getting away all of a sudden, when we were faster just moments before?
GM: That is absolutely correct. Strange is it not? Doesn’t it make you wonder how it did that?
Player: *paranoia intensifies*

[b]Don't pull your punches, telegraph them.[/b]
In other words let actions primarily have predictable consequences. This isn’t necessarily the same as instantly punishing a player for getting caught up in the heat of the moment. For example if during a chase scene someone forgets what floor they’re on, a convenient haystack is much better than a splat sound. Provided that the building’s height is or was not a plot point and/or has not been repeatedly and clearly stated to be that tall. Because that is what this bit of advice is all about. When players for some reason or another clearly insist on doing something dumb that they have been given warnings about, don’t be afraid to drop the hammer. Trying to take on a dragon ten times their level, using a bomb as a melee weapon, or pimp-slapping that one really important noble of importance are just a few examples of mistakes that ought to result in severe consequences.