So ... I don't go as all out as I might, if I had walls in our library. It never fails but some of the people going through the hallway spoil the effect.
But I don't let it stop me entirely. For a couple of years now I have offered 15 to 20 minute "Slightly Spooky Stories" the week before Halloween.
I have a string of orange lights that we pretend is a campfire. A silly flashlight with buttons for screams or wolf howls. A CD of creepy noises. It's all in fun. The most popular (and creepiest) stories have come from Robert San Souci's Short and Shivery books.
This year I think I will add some of Neil Gaiman's read aloud of The Graveyard Book. And something from Half Minute Horrors. Some of those give me the wilies!