At the end of the day, the aim of all this is to have fun, after all.
Brief, non-laboured points are probably going to work best. If things get into deep discussions on something specific I will probably split them off into their own threads to keep this one running smoothly.
I'll kick off then:
1) GM's are basically god. If you want something to happen in a certain way, it should happen in that way. If you have to fudge dice rolls or break rules to get to that point, so be it.
2) On the other hand, the point of a game is for everyone to have fun, so the GM has to put the enjoyment of their players slightly ahead of themselves. Remember, in nearly all games there are more players than GMs, so if the only one having fun is you, the balance is off.
3) Dont break too many rules. If you think a rule in a game is totally stupid, talk to the players about it, change it as necassary, then make sure everyone understands the change. If in doubt, dont change anything.
4) Its a game, not a simulation. You will never get a fully realistic depiction of everything, and even if you could it would be fucking dull to play. Abstractions are necessary to keep a game flowing.
5) Dont railroad players. Players should feel like they are in control of what they are doing. If they are flailing around not knowing what to do next, give them hints, but if they are about to do something stupid, let them. They wont do it twice (hopefully!) and some of the most fun RPG moments come from when things go fubar.
6) Prepare, but dont overprepare. If players are likely to go to a location, work out the layout of the place and put some basic stats together for it, but you dont need intricately detailed maps of every location ever, nor do you need full character sheets for every NPC. Indeed, if the npc is only ever going to be talking or something similar, you probably dont need stats at all.
7) Sort of following on from the above, have some pregenerated stuff for emergencies. Make some stat blocks for generic goons of varous types, so if you run into a situation where your players do something totally unexpected, you dont have to put together stats for things on the fly. That rarely works well.
8 ) Understand the maths. For instance, the shadowrun system works in such a way that on average one in three dice come up as a hit. So you can quickly work out average dice rolls: a dice pool of 6 will average 2 hits, for example. This can be tremendously handy working out how difficult things should be. Other systems will have similar tricks: work out yours, and work out what it means.
Im sure I'll come up with more. Feel free to chip in/comment/argue
