I'm not sure how much others would agree with me on this, but I would say not showing the monster when there is one.
A little bit of that in a horror movie can raise suspense, but reading about it, trying to envision the monster, I just find much more hair-raising in written form than visual. I guess because then you can let your imagination run away with you better.
Edit: Oh, and this is doubly the case when the movie doesn't have good special effects. The monster never looks cheesy in your mind!