When used correctly, metaphors are effective fiction writing tools. They are one way to vary language and liven up prose. Also, like shorthand, they can convey a picture or a meaning instantly, with few words. Of course, like most literary devices, metaphors bomb when used incorrectly, confusing the reader or drawing attention to the writer's lack of skill. The two most common traps to be aware of when using metaphors are the cliché and the mixed metaphor.
Expressions like "the calm before the storm," "Mother Nature," or "he's a rat," have been used so often that they're now considered clichés. Unless they're important to the tone of your work, it's usually better to avoid clichés. If you can't come up with the more original language, it's often better to forego the metaphor and opt for straightforward description.
Another common problem with regard to metaphors is the tendency to mix them or overwork them, usually from careless thinking or over thinking. The result brings together two images that simply don't make sense together. In "Examples of Metaphors", I give the example "Our keyboard will teach your mind's eye to play by ear." Here, the speaker has mixed two metaphors, leading to nonsense. A "mind's eye" can't play anything, and certainly not "by ear."
In Rules for Writers, Diana Hacker gives the example, "Crossing Utah's salt flats in his new Corvette, my father flew under a full head of steam." She explains that "flew" suggests an airplane while "full head of steam" suggests a train. A little thought and a small edit results in a clearer, more satisfying sentence: "Crossing Utah's salt flats in his new Corvette, my father flew at jet speed."
The book English Made Simple provides a particularly muddled example. Notice your own reaction as you attempt to read this paragraph:
The age of the atom has transfigured man, but the transfiguration has been from spirituality to materialism, exactly the reverse of the counsel of the ancient prophets. Today, man surges forward in his march of mind, but his soul is mired in an abyss of false yearnings. In splitting the atom, man has split his own being and lost the unity of his molecular whole.
It's hard to get through, isn't it? As the authors explain:
The writer begins with an interesting image — transfiguration — but shortly afterward loses it in an unrelated figure about a march of mind. Furthermore, he gets his metaphors tangled when he gets a soul mired in an abyss. Finally, he returns to the atom, but the original image of transfiguration has now entirely disappeared, and the reader wallows in the murky depths of molecular whole.
Notice how much easier it is to focus on this selection:
The atomic age has halted man's spiritual development. Intellectually, he has surged forward, but his ideals and his values have stood still. If man's moral being regresses, atom splitting may prove an illusory advance.
If you're ever in doubt as to whether or not you've pulled a metaphor off, it's better to nix it altogether. Used improperly, you'll wind up looking foolish and distract your reader. Remember, your overarching goal is to communicate. Sometimes this means putting aside your ego in favor of plain speaking.