Introduce a purposeful ghost. If the scary story is supernatural and contains a ghost, this maintains the reader's attention. According to Susan Hill, author of the spine-chilling story, "The Woman In Black," writing in the Times newspaper, "the ghost must certainly want to do, say or bring about something---or the story fails." Your ghost may have been murdered and want to seek revenge, or reveal a sinister family secret.
Inject atmosphere. Atmosphere, Susan Hill notes, is contained in place and setting, and often reflects the fear and danger experienced by your protagonist. If writing a classic story, set your story in a Gothic mansion, have the action unfold on a stormy night, or in a snowstorm. If writing a modern story set in a contemporary apartment building, for example, find something sinister to say about the building. Rosemary's Baby, written by Ira Levin, was set in a contemporary apartment building but it had a nefarious history and creepy tenants.
Omit excessive detail. Some of the best scary stories skilfully leave it to the reader's imagination to fill in the blanks. It is often what is hidden, or merely hinted at, that sends chills down the spine. Don't introduce your ghost or murderer in the first chapter and tell the reader his entire history, as well as his evil intentions. Your reader will lose interest. Keep your reader guessing, be subtle and your reader will be intrigued enough to read on. Introduce a surprise twist at the end. The most effective scary stories always have an element of surprise or shock at the ending. Think of Stephen King's "Carrie."
Engage your reader's sympathy. If you introduce sympathetic characters, your reader will identify with them, and care about what happens to them when they are plunged into a maelstrom of terror. Write a personal history of your main protagonist, where he went to school, his first love, likes and dislikes, lovable quirks, even if you don't include all these details in your story. It will make the character more believable to you and, therefore, more believable to your reader. If you make your protagonist seem real, your reader will be more convinced when her co-workers turn into flesh-easting zombies.