Foreshadowing--a subtle hint, warning, or indication that a specific event will occur in the future.
Although foreshadowing is often used in romantic suspense and mystery, it has its place in other genres as well. By giving hints about a future event in the story, you can build tension and anticipation. But be careful about obvious clues. If the reader already knows what's going to happen, she has no incentive to continue reading.
Red herrings can be effective under some circumstances, but don't neglect hints at the truth. Misleading the reader can backfire.
Also be sure to follow through when using foreshadowing. Implying someone in the book will die and not wrapping up that loose end creates the perfect opportunity for a reviewer/reader to criticize your story.
What makes effective foreshadowing?
Subtly is key. By using two or three almost invisible suggestions, the event can be planted in the reader's subconscious mind. The hints may or may not connect the dots. Sometimes, the reader will ask, "How did I not see that coming?". Other times, that gut feeling kicks in. The reader then gets that I-knew-it-was-going-to-happen satisfaction of correctly guessing the outcome.
Animals are often more intuitive about natural disturbances than humans. If a tornado will hit your small Midwestern town, the main character's cat might act strangely from the shifts in atmospheric pressure. I've often seen sudden behavior changes in my cats when a high- or low-pressure system starts moving into the area. Use these survival instincts to put careful suggestions into the reader's mind.
Use a dog's well-developed senses of smell, hearing, and sight to hint at something foreboding. While the hero thinks the whining and pacing means his dog simply needs to go out, the animal might actually know the villain is lurking outside or that Timmy fell in the well. Sorry, I couldn't resist a little Lassie humor! :)
A serious discussion can foreshadow the death of an important role model. Even an off-hand comment can come back to haunt your characters. Does the heroine always take the stairs? Perhaps, the reason isn't only because she prefers the exercise. Could she have a fear of elevators? That bit of information could lead to her getting stuck in the contraption with her hunky sworn enemy.
Avoid adding too many indications, especially strong ones, and giving away everything. Go for the "Aha!" moment instead of the predictable plot.
Romance...With A Kick!