[DeadParrot]

Be consistent on how to resolve things. I have played with GMs that resolve combat and most skill type situations with die rolls but when a Charisma type skill like diplomacy is attempted, the GM suddenly demands that the Player make a good real life diplomacy speech. Players with poor verbal skills are often penalized in such situations even if the Character's abilities are good.

Avoid the use of tightly scripted adventure paths. The few I have played in quickly felt like I was an actor in a play having to follow a predefined script instead of a character with a fair amount of free will.