Read horror voraciously. By studying how other authors create a scary mood, you will be more able to internalize their techniques. Note which techniques make you uneasy or frightened and which leave you unmoved or strike you as silly. Although this sometimes has more to do with personal taste than the author's skill, discovering your taste as a reader of horror is important. Your own sense of whether a scene you write is frightening is the only barometer you have for that scene, so personal taste is an important part of the process.
Create detailed, realistic characters. Consider their motivations, their goals, their personal histories, their likes and their dislikes. Avoid stereotypes, or add detail to a character who might otherwise seem stereotypical to make him fresh. If the reader believes in and cares about your characters, she will be frightened if she feels they are in danger.
Establish a setting which will feel somewhat familiar to an average reader. A crumbling haunted house on its own isn't necessarily scary, because few readers would ever willingly enter it. A house that looks like a quaint, welcoming townhouse but turns into a crumbling wreck the moment the hapless character steps inside is more frightening because the reader can picture himself stepping inside instead of the character.
Include subtle details rather than going over the top. A few drops of fresh blood in a room that no one has entered for days is unsettling; a waist-deep lake of blood is ridiculous. If you want to include something extreme such as a lake of blood, build up to it slowly with more understated signs of wrongness. First, the character has the vague sense something is off; then, a few paragraphs later, she notices a faint, cloying, metallic smell in the air and wonders if that was what made her so nervous; then, a paragraph or two after that, she feels that the floorboards feel spongy and weak, as though they've grown wet and moldy; and only after all that buildup do the floorboards give way and dump her into the waist-deep lake of blood in the vampire's cellar.
Remove your character's defenses and escape routes. Your readers will not be frightened unless there is a sense that your character is in danger. They will be more frightened for a character who is alone, defenseless and has no way of calling for help than one who is armed and surrounded by friends. The exception is if you establish that the character's defenses are useless in the situation. The character may think himself safe because he is surrounded by bodyguards, but if the reader knows that a werewolf is loose and those bodyguards do not have silver bullets, she will still fear for your character.