Tips for Writing Horror

Topic and Source Material

Death: Since we were children we all wondered, suppose the dead really aren't dead? We can play on these fears -- along with other primitive fears of vulnerability, abandonment, the night, the dark, the forest.
Nightmares: They are rich material -- Frankenstein and Dr Jekyll and Mr. Hyde evolved from dreams. Classic stuff -- vampires, ghosts, monsters, werewolves – is always good.
Basic childhood fears: Childhood vulnerability is a good place to begin writing horror -- the creatures we imagined under the bed, in the closet, basement, etc. Childhood fears are more about abandonment and vulnerability.
Basic adult fears: Grownup fears are more about death, being out of control, and all-powerful forces that we can't stop.
True stories: Real hauntings, real mysteries – familiar settings, legends, folklore.

Constructing a Good Scary Story

Tone is essential to horror -- it must start in the first paragraphs. Since you are going to write a short story, don't delay: set the mood in your setting using shadows, weather, old buildings, etc. to set up an atmosphere of gloom, danger, sinister possibilities. Use words that carry an air of mystery or “negativeness.” Let horror, or spookiness, permeate every part of story.

There also has to be some sense of reality established, perhaps in the setting, in the human characters, in the scope of the threat; without some tangible, believable element your readers can't trust you, or follow the story.

Pacing and Suspense: This is really difficult in a short story, but try to delay the horror a bit -- tension is delicious -- but keep pointing towards menace: have small things go wrong, misplace an item or two, make the familiar seem strange somehow. Often in fiction there is a hierarchy of sees, knows and believes in the evil. First, it's the cat who knows the place is haunted. Then the kids start having nightmares, figuring it out. Then the authority figures catch on. So you might want to start by threatening the most vulnerable characters first.

We want to keep our scary tone or mood fairly constant, but we don't have to keep the action jamming constantly. Tension and suspense are created by the unknown -- by a terrible waiting.
Anticipation, holding back the outcome, is hard in a short story. You have to really rely on details to create suspense. In short fiction there isn't room to overload with scenes of horror; you have to be more discriminating. The threat must be real, but you probably won't have a chance to stage it until the end.
We don't want a constant stream of frights, monsters, conflicts. We need some normalcy, downtime, sequel, WHILE we keep the suspense, while we know that something monstrous is lurking. Don’t forget to build a little of that in as well.

Monsters! Start with a fully fleshed-out main character and proceed with at least a bare outline of where you are heading.

If you're going to work with a monster, it must still be understandable and real in human terms -- despite his supernatural powers or incredible evil, there must be some human element there -- like rage, empathy (Frankenstein), pity, etc.
Most beginning writers should stick to one or two monsters, sources of terror. Don't bring in an army of ghouls; it's usually overkill. One awesomely threatening creature or circumstance is usually plenty, so concentrate on one major threat. If the human beings are totally outnumbered and helpless this will also kill suspense.
We usually don't expose the full powers of the monster, the full force of evil until the very end. But along the way we REVEAL bit by tantalizing, terrible bit just how capable he is of ruining a picnic. The reader is learning about his evil ways along with the characters. Remember that revealing too much, too soon, robs the suspense -- suspense = delay.
How evil, weird, powerful should the monster/creature/threat be? Ideally, even if he's fabulously evil and powerful, there is usually a chance that a human can somehow survive him, face him, do him in.
Your monster must also be somehow removed from the ordinary world – isolated or alienated. This is a very common theme in all literature.
Monsters don’t have to be monsters at all: evildoers like human stalkers or intruders are often more real and therefore even more scary than supernatural baddies.

Other Helpful Tips

Don't allow ALL the characters to believe in the monster; have some skeptics in the crowd.
Keep dialogue tight and believable.
You can use humor from time to time to relieve tension and make it realistic.
Don't use gore for gore's sake -- psychological horror is sometimes much more satisfying. In fact, if you are going to use gore, you better make sure it’s very well written with great vocabulary and style. Figurative language works well here!
Isolation is a good technique to slip in. No one is around to help.
The weather is another factor; storms are an ancient technique to mess up someone's life.
Remember to use obvious details like nighttime and darkness. We are simply not as brave, strong, sure when we don't have easy visibility.
Terror is the fear that harm will come to oneself. Horror is the emotion one feels anticipating and witnessing harm coming to others who we care about.

Source: Page Morrell, Jessica. "Writing Horror Fiction: A Writing Class Transcript." iVillage. 2003 <http://www.ivillage.com/books/expert/writecoach/articles/0,11872,167255_45899,00.html>.