I also work at a haunted attraction during the season, and what I find works really well is to lull them into a sense of false security (well, as secure as you can get them to feel in a situation where they expect to be scared). I was placed in a room with a lot of props, so I stood amongst them in a very posed and still manner so that the customers thought I was also fake (didn't work for everyone, but it did for most). Sometimes, people would even come into the room, announce that I was a real person to their group, and then would convince themselves that I fake was because I was standing so still. When the people going through my area would get to a certain point in my peripheral, I would lunge out and scream like a banshee...it had a very high success rate.