I'd like to pass on some suggestions from Brian Klems article: Five Ways To Make Your Novel More Suspenseful. This was in an e-mail I received from Writer's Digest Magazine. I found the article to be what I would call a keeper. For the sake of brevity, I'm only paraphrasing what I read. If you'd like to read the entire article, please click here.
#1. Turn up the sensory detail.
Suspense is sustained by the absence of anything terrible happening, and the continued focus on detail. By focusing in on the little things like a car back-firing, the hiss of a cat or anything that would keep the reader holding their breath, you are building the suspense.
#2. Turn down the velocity.
Slowing down time increases suspense.
a. Complex sentences. Stay away from the short sentences that speed up the action.
b. Internal thoughts. Let the reader hear what your character is thinking.
c. Bring the reader as close as possible to the scene. Let them experience the tension of the sequence first-hand.
#3. Modulate suspense.
Building suspense takes time. Break the tension by having something happen that advances the plot or provides a moment of comic relief.
a. Insert a pause. A telephone rings. One of the characters cracks a joke. Remember, in real life, humor is used to ease tension.
b. Reveal something that seemed menacing to be ordinary. A scary shape turns out to be the shadow of a tree in the moonlight. A hand on the shoulder of the protagonist turns out to be his best buddy.
You can use this technique to give your reader a respite, then continue to crank up the suspense to keep them hooked.
#4. Foreshadow rather than telegraph.
Creating a suspense sequence that ends harmlessly is a good way to foreshadow something more sinister that happens later on in your novel. Be careful to foreshadow and not telegraph: giving away too much too soon is guaranteed to ruin the suspense.
The line between foreshadowing and telegraphing is subtle. When you insert a hint of what's to come, look at it critically and decide whether it's something the reader will glide right by, but remember later with an Aha! That's foreshadowing. If instead the reader groans and guesses what's coming, you've telegraphed.
#5. Always end with a payoff.
You can have a suspense sequence early that ends with nothing more than a harmless tabby padding off into the night. But as you near one of your novels end-of-act climaxes, the suspense sequence should pay off. The payoff can be an unsettling discovery of evidence of a crime: finding a dead body, bloodstained clothing, a weapons cache, or that the floor of the basement has been dug up.
The discovery might reveal a character's secret. Finding love letters might reveal a hidden relationship between two characters.
Or, the payoff can be a plot twist. The bad guy confesses, the sleuth gets attacked, or locked in a basement, or lost in a cave, or the police show up and arrest the sleuth.
I hope these ideas have given you food for thought and have stirred your juices around enough to check on the suspense in your novel. The main thing is to enjoy the process. And write!