To these guidelines for GMing, I would like to add one more:
Instead of planning what the characters will do to the plot, plan what you and the plot are going to do to the characters.
So what does that mean? It’s an expression of the old wisdom “Once players get involved, nothing will go as you planned,” phrased in a way that focuses on what to do about it. And the what is: rather than planning out “the characters will go here and talk to this guy and then go fight whatever,” plan out “this guy will come to talk to the characters and try to convince them to fight whatever he needs them to fight.” And then maybe a fight will happen, or maybe it won’t. Or maybe the whatever will attack them by surprise mid-conversation.
See the difference? Good!