Heroes of Horror has some good horror guidelines.
As a long-time WoD Storyteller, the most effective tactic I've used is "don't show everything." Strange noises the PCs hear, but they can't tell what caused them. Glimpses of things, just a shadow crossing a doorway for example. Hint at the horrible and terrifying, but don't SHOW it until it's ready to rip the PCs faces off.
Seemingly innocent things possessed by great evil work too. Demon-possessed children, ghosts inhabiting stray dogs or a housecat.
Use metaphor and omens, too. A murder of crows hanging around outside of a window can work wonders. Especially if they're staring directly at the PCs, making no noise whatsoever with a malicious intelligence in their eyes.
Darkness is good. Make sure to describe it not just in the visual way. Muffled noises, trouble breathing and a general sense of being lost even though they know where they are. Lots of people feel claustrophobic in the dark, even in areas they know are wide open spaces. Play up on this. So what if the PCs have light cast? The darkness is so "thick" that it seels to swallow up that light, and, just on the edge of that safety zone, where the PCs can't see, something shambles. Remember that fear of darkness is a very primal thing, use it. Light is a comfort zone, invade that comfort zone and lessen it.
Use threats. The threat of demonic possession or of a painful, slow death can be far more effective than the acts themselves.
One of your best tools will be description. Describe the thick layers of dust, the darkness, the shadow in the doorway, the noises, the cobwebs...even if it seems like a cliche', it's going to work. And don't shy away from gore; as long as it's not completely overdone a gory scene can work wonders. Also, practice the art of under-describing. There are times when you should leave blanks that the players' imaginations are going to fill in and their thoughts will often be worse than anything you can come up with (yes, I know this seems like contradictory advice, but the trick is knowing when to describe and when to under-describe).
Alienation, paranoia and loneliness work wonders too. Split the party up somehow. Breed paranoia and distrust between the characters. Trust in one's allies is a safety net..remove that safety net (or at least lessen it to a degree). Take the characters (and, to some extent, the players) out of their comfort zone.
If you're gaming face-to-face (I know, you're not, but just in case you ever do), lower your voice volume by a bit and deepen it subtly. If the players have to strain just enough to hear you, it can build tension.
Try to utilize at least three senses when describing a scene (this is usually a good rule of thumb IMHO for any scene, not just horror ones). Sight and sound are obvious ones, but what about the prickly feeling as the hairs raise on the back of the PCs necks, or the smell of something undescribable lingering in the air which leaves a bad taste in the characters' mouths. If you want to combine some of the info above, try NOT describing with visual cues; describe how the area smells, feels or tastes and let the players fill in the visual details on their own.
Remember that hope is one of the greatest tools of those in situations of great fear. Use that to your advantage. Let them think they're safe, only to have that safety and hope torn away from them at the last minute.
Completely separate the horrific situation from the mundane situations. This creates a great contrast. Make everything seem happier and brighter when not in the situation, then when they get themselves where you want to start sending shivers down their spine, make sure they're in a completely different situation; things get darker and more ominous and they should feel like they're cut off from the bright, shiny, happy world. All they have to rely on is each other, and really...can they still trust the person next to them?
Hopefully these suggestions help.
EDIT: One thing I forgot. Humor. If the players start trying to inject some humor into the situation, you're doing things right. Why? Because they feel tense and are trying to release that tension. You want this in a horror situation.
