This is a review (of sorts) of a book that shall remain Nameless, broken down into all the things it did that you Really Shouldn't Do. Part one and two are here and here.

Foreshadowing is a subtle creature. Handled poorly, it can telegraph your plot points for miles, or leave your reader utterly bewildered as they wonder where the galactic spaceship fits in 18th Century England. It's an area where the author really needs a second pair of eyes to skim the text and point out what was too obvious, too obscure, or just downright silly.

Foreshadowing is like scattering echoes of major plot developments into the text before they happen. It's a way of setting up the rules of the world, and telling the reader the kind of story they can expect. If a plot element is foreshadowed correctly, then when it occurs it will feel like a 'natural' progression of the story, as if it was the only path the story could take, even if the reader wasn't expecting it.

There are a number of ways of foreshadowing. Metaphor and mimicry are the most common, and the most abused. Metaphor uses imagery or symbolism of the plot point - for our high-tech police ghost story, perhaps a halloween ghost prop, or a sheet floating in the wind - something that the reader responds to subliminally and associates with your chosen plot development. Metaphor can be extremely effective, but it has a high risk of obscurity and anachronisms or other errors. The word 'subliminally' is key, there - foreshadowed plot metaphors can't be something that the reader has to sit there and work out: for one thing, they won't bother, and for another, the time they're spending figuring out your metaphor is time they're forgetting your story. 

 Mimicry is when a minor event has strong elements of the major one. Our high-tech police officer encounters some halloween kids dressed as ghosts, or is momentarily spooked by something that turns out to be ordinary (okay, I'm not winning points on imagination, but you get the drift). This has the advantage of essentially hitting the reader over the head - it's rare for this kind of foreshadowing to be too obscure, but it does risk being just the opposite. If it mimics the plot development too closely, then your plot direction lights up in neon, and your reader rolls their eyes when your grand event is revealed.

Another method of foreshadowing is one that I recommend you never, ever use. It's the tactic Nameless chose to go with, and one I think I'd never encountered before - I'll call it 'dialogue foreshadowing'. Can you guess? It consists of characters telling each other what's going to happen in the plot.  Not only did that make the plot eye-rollingly obvious, it made for very painful dialogue.

Whatever method you choose, you can't afford to not forshadow major plot elements or world rules (another Nameless mistake). Without foreshadowing,  your story won't flow or feel cohesive. It won't have the scaffolding inside that makes Act 1 mesh with Act 2 and then both unerringly lead to Act 3 - instead, what you'll end up with is a Bunch of Stuff That Happens, which is rarely fun to read.