Acknowledge the devil in the details. Convincing narrative of any variety is supported by a prevalence of well-chosen details that describe both the characters and the setting. Think of each vibrant detail as a painter's brush stroke. You increase the intricacy of your story as you layer detail upon detail in the tale. For instance, if a door is critical in your story, you might linger on the feel of the doorknob, the chipping paint at the edges of the frame and the eerie creak it sounds when left ajar.
Structure important points to your story's plot before writing. If you understand when and where events take place, you can better plan how those events unfold. Steer clear of over-preparation, however. Your writing might benefit from spontaneity and the spur of the moment.
Tantalize your reader with the art of chapter breaks. Ending each chapter or section of your story with a new event or happening that builds upon what's come before can stoke a reader's interest. Lay a new detail or plot point at the conclusion of a chapter to fuel your narrative's energy.
Play to your audience's tastes. When a writer understands characteristics of her readership, she can tailor a story to those specific qualities. A horror audience likely expects mentions of blood, violence, nighttime and the supernatural to surface in their reading. Don't shy away from judiciously incorporating images and scenes that reflect these ideals throughout your horror narrative.
Sparingly use the element of surprise. A sudden inclusion of something new can instantly inject tension into a story. Surprising a reader too frequently, though, can be seen as a stunt and lead to his desensitization.
Maintain consistent mood and style in your writing, especially for the horror genre. A sudden diversion into florid prose in an otherwise tautly composed story can quickly deflate tension and disinterest your reader.