Include foreshadowing in dialogue. Conversations between characters can be very revealing, because you have a chance to include emotional reactions to situations that help the reader become invested. For example, giving a character the preliminary physical symptoms of an illness and having her comment on the effects gives your readers the opportunity to sympathize and make possible connections with other plot points. Simple comments like "I've been getting these headaches for the past two weeks" might be a hint at future problems.
Use symbolism. Symbolism, or using an object to represent an idea or person, may not be an obvious form of foreshadowing, but it can be very effective. For example, in the novel "Jane Eyre" by Charlotte Bronte, Jane is locked in a red room as punishment after being unjustly accused of misbehaving. Later, whenever she feels isolated or unfairly treated, she mentions a red room.
Plant a "loaded gun." The stereotypical "loaded gun" is a popular symbol for trouble to come, but it can be placed so discretely that the reader barely registers it, and as a result, is completely surprised when it shows up later in the story. It doesn't have to literally be a gun; for example, it could be a strange package that's delivered and remains unopened for much of the story, or a character who constantly forgets to fill his car up with gas. These plant the seed for future events.