A reader can't feel suspense unless he properly identifies with the characters in the story. The characters need to be pleasant and likable, possessing qualities to which the reader can relate. They need to ring true as well, acting as plausible, flesh-and-blood human beings rather than cardboard cutouts. When the story places them in danger, the reader needs to understand their thought processes, to feel their fear and to connect with them in their moments of peril. Plan out your characters carefully and prepare scenes that help establish their sympathetic qualities.
The reader's imagination can fill in the details far more vibrantly than any writer. Whatever she conjures in her mind is bound to be more terrifying to her than the things you put on the page. Accordingly, a suspense writer must learn to evoke fear without being blatant about it. Use your prose to give a basic impression, then back off and focus on the emotions of the person in danger rather than the facts about the danger itself.
Excise certain details during suspenseful scenes. Give hints about the threat without describing every last aspect. An unknown shape in the corner represents a palpable, but nebulous danger on which the reader can imprint any terrors he wishes.
Suspense novels often overlap with mysteries, and their shared sense of the unknown provides both genres with a unique spark. You can aid in that impression by periodically providing pieces of misdirection to keep the reader guessing. Don't be afraid to let characters lie and work on generating false leads and red herrings to throw readers off the scent. If they know who the killer is from the beginning, it's apt to utterly destroy any sense of tension. When deploying such tactics, however, take care not to cheat: that is, to use deception or misdirection in a way which prevents the reader from possibly guessing the outcome. For example, don't introduce new information at the last minute that renders all previous assumptions wrong, for example, or present a flashback as the truth only to later reveal it as a complete fabrication.