Think about a horror movie for minute.
Pacing: most horror movies are pretty fast paced, and move from scene to scene quickly. This can help keep viewers (or players tense). But sometimes they slow dow
Mix it up: there isn't something gross in ever scene. Sometimes a chase is a chase and you get away. Sometimes there are moments for character development (not horror's strongest suit, but sometimes it happens).
Leave things to the imagination. Hitchcock was great at this, giving a different perspective and letting the viewer think about what the scene actually looked like. Give your players the silhouette in the shower and let them describe the gore.
Set the mood. It's no coincidence that a lot of horror happens in the dark, when it's stormy, with strobes etc. If you're not fortunate enough to be playing during a storm, make sure your descriptions of the environment are vivid, and dim the lights (if possible). Don't be afraid to have someone that's not playing suddenly cut off (or flash) the lights at a tense moment. Bringing the physical environment into play.
Build to drama. Someone might die at the beginning of a film, but a viewer may only have vague details. Let is simmer for a bit and build to your climax. Whether your drama builds mechanically (say like in Dread) or story wise (hopefully both). Don't rush right into the blood and guts. Drop subtle hints at the beginning, and then drop the hammer towards the end of the scene.
Most of all (like with any game), know and watch your players. Keep an eye out for signs of boredom, tolerance etc and use those moments to quicken the pace and kick the plot in the direction you want to take it. If you find that you'reyour players aren't as scared as you think they should be at a given moment, bump up your description, your pace, or give them a fright (have a scare prop or two on hand just for these occasions).