Next to Christmas, Halloween is a very large commercial holiday here in the states, and more Americans are desensitized to being scared or the violence because of the movies, but here are a few tips that might help you. Remember there are obviously people who are dying to be scared, so go with that.
If you can have a person dressed in a costume that can stand still while the people walk into a room, and then move when they are not looking - this will scare most people.
Use horror icons like Freddy, Jason, Leatherface, and others that might people might already be afraid of. Evan a clown from the movie IT can make people run if they have sharp teeth. People are afraid of the sound of a chain saw.
Dark small rooms are great to play on peoples claustrophic feeling. Hanging items from the ceiling that will touch their head, or having sticky stuff on the walls to touch, or walking over crunchy dried leaves, or even a rotten smell will get people to think about what might be in that room with them.
Sounds are great to add that eerie ambience, or a fog machine, or a special light effect will make a difference. These days the computer can even be useful tool.
You might even want to think of a theme or idea for your backdrop, like an outside grave yard, or butcher's shop and don't forget to have corpse.
What is the ultimate scare? Answer: Getting someone so unnerved in advance that they don't even want to go inside because they are to scared.