The main thing I'd recommend with a rattlesnake, is make sure you have the rattle sound effect. Once you have it, you can actually use it a few different ways.
1) Definately use the sound of the snake when you have it "strike". You want to make sure the customers associate the rattle sound with the snake.
2) You could use the rattle sound to foreshadow the snake. It's a very recognizable sound, and just knowing the possibility of a snake coming up can build the tension.
3) Use the rattle somewhere again down the trail. Once they have seen a snake once, additional rattle sounds will have them worried about seeing it again.