Rules for GMs
Rule #2: Improvise! It sounds cliche, but too many GMs don't do this. They stick to what they have written and if the players don't cooperate, they're lost. I played in a GURPS game once where the GM started the scenario with a run-in against the game's main villian, a powerful telepath. My character whips out his gun and shoots him in the head. End of scenario. The GM throws a fit. Instead of improvising a way to keep the character alive or coming up with a new villian who happens to have similar abilities ( a brother maybe?), the new campaign ends one hour into the first game. This is why I write very little down. I try to have names for NPCs, purposes for them in the scenario and a general theme. Everything else is usually made up on the spot. Different GMs will be more comfortable with different levels of improvisation, but if you are GMing you have to always be ready to change the story or a character on a moments notice. Major villain about to get shot in the head during trhe first game? It's a good thing one of his assistants is a telekinetic and can stop bullets with his mind. What? Was that really something I planned before-hand? Of course it is. ;)