I've learned that, no matter how little I prepare ahead of time, my players will only get through half of it by the end of the night.
I've learned that it's the little things and cheap tricks which stick in my players' minds. (For instance, in CoC, I occasionally startle them during tense scenes. I do little things like slamming my hand on a table, subtly throwing a coin at the wall behind them, or covertly play a sound file on my laptop. Tonight, I'm going to start playing insane pipe music on a computer a few rooms away with a 3-minute silence gap preceding it...)
Finally, I've learned that I don't need to know exactly where everything is going in a campaign. If I throw in a random spooky occurrence, NPC, or tiny reference, all I need is a note to myself. I can work it in later on down the road, making me look like I had brilliantly planned it that way all along. This gives a fantastic illusion of consistency that's flexible enough I can work with whatever the players want to do.
-O