I posted this as a comment on another thread, but felt it was worthy of it's own post.
There are some rules a gm should always follow. They are rarely part of any game system. Here are my rules for being a gm.
1> Thou shall go to any length that will cause your players to have fun, including chucking the rule books out the window. 
2> Thou shall be aware if your players do not look like they are having fun.
3> Thou shall be entertaining.
4> Thou shall never 'seize control'. (railroad). The game is controlled by the players. You are merely a catalyst/interpreter.
5> Thou shalt not smack the players with anything they can not solve/handle, unless recourse is given later. 
6> Thou shalt not pick on one person above another.
7> Thou shalt provide equal time to all players whenever possible.
8> In this equal time, it shall include equal effectiveness and oppurtunities for enjoyment whenever possible.
9> Thou shall provide choices to the players that allow them to illustrate that they do indeed have control.
10> Thou shall listen to players who express complaints as per rule #2.
11> Thou shall ignore all complaints or critisicms from one party that seem to interfere with rule #1 for everyone except the complainer.
12> Thou shall give creedence to any complaint that comes from 2+ players, as this is directly tied to rule #2
13> Thou wil never allow npc's to solve problems for players. 
14> Thou shall never allow rules to interfere with rule #1
15> Thou shall never allow ego, attention hogging, storyline, bad mood or anything else included or not included within these rules to interefere with rule #1.
16> Thou shall modify all of these rules that conflict with rule #1 or add any rules that help insure the execution of rule #1.
 This can in fact be a great deal of fun with some particularly frustrating game systems.
 Smackng them down and capturing them in ambush is ok, becuase the sweet feeling of vengence when they come back and thwomp the villians is oh so sweet.
 This rule is somewhat pliable, in the case of setting up really powerful heroes who will need to be rescued later, or bailing out players out of a fire they didn't have a chance to stop.