I'm going to add something that is obvious (for all things) but is not nearly enough mentioned, I certainly have not seen it mentioned on these boards ever.
Getting enough rest (as a GM) the night before really helps one with attitude, patience, clarity of mind and the ability to recall ideas you've been running through your head much easier.
I'd say, for me, it is probably more important than having anything prepared. I run through ideas while I'm driving to and from work, or just before I go to bed or when I'm brushing my teeth or showering...etc, I do not need masses of notes.
But nothing gives me that peace of mind before a game like a good night's sleep.
Has anyone else found this to be invaluable? I just feel I run a better game when I'm rested.