I find myself scaring my PCs in several different ways all dependent on what I play up as my strength, description. Description can build up mood and atmosphere, hint at things and betray or meet those expectations, and entail in a few choice details the worst of an encounter.
My latest kick is in introducing the PCs to sudden, grotesque and frightening scenes which break their normal expectations and throw them off as characters and players, in essence shock them. Shock can be a powerful tool, especially when coupled by a rapid entry into combat or a chance for the PCs to interact and deeply react to a scene.
I'm pretty good at obscene description, but used too much and it loses its effectiveness. That's where mood, atmosphere and build up come in, dotted with moments of genuine light or humor to sort of refresh and allow for new tension to build.
Still, if you're creative enough you can get a lot out of the shock tactic. This also hinges on the player maturity around the table and your own comfort in introducing said horrendous elements. And I can't stress enough that a little detail can go a long way if its the right detail. Luckily with a mature group willing to be exposed to "vile" content, this is a fun and effective method. It's almost become a thrill for my PCs to see what the next demon lord and his or her lair will look like, surges of excitement, fear and a bit of horror-movie-like anticipation. Of course, sometimes it's just down right nasty and depraved, and that works well too.