A dead metaphor or other overworked bit of comparative language can break the reader bond, Gardner's "fictive dream" state, by boring the reader. Cliches fail to engage the imagination, and they expose weak spots in the writer's thought process. They will still creep into first drafts -- it's inevitable, especially during fast drafts. NaNo, anyone? But it's important to cull them during revisions and replace them with more apt language.
Brainstorming a new metaphor is not all that difficult. Let's say you have a real clunker, a horrid, turgid, dull simile that would make any worthy reader groan: "Her swan-like neck" used to describe a romance heroine in an intimate scene. Ugh. I cringe. Bet you do, too.
There are at least two ways to brainstorm new descriptive language there. The first is to tackle it from the underlying concept. In this case, the swan-like appearance of the neck refers to length and slenderness rather than to feathers or whiteness or twisty flexibility. So think about other things that are long and slender. Don't worry about whether they make sense. Just list as many as you can think of:
rebar, downspouts, those long skinny balloons, string, spider legs, a garden hose, kudzu vines, hanging moss, icicles, a silk scarf, butterfly feelers
As you see, the results can be mixed -- which is fine, because this kind of creative flexing will never hurt you, and because some of these images might be useful later even if you don't use them now. Some of them lose the sense of flexibility (rebar? honestly, it was the first long skinny thing I thought of, and that is just SAD), and some of them connote things you might not want to incorporate (icicles = cold, frigid, immobile). But eventually, you'll hit on something that might work and won't bore the crap out of your readers. I'm kinda grooving on that hanging moss image as something very organic and natural and tactile, but it has connotations that might prohibit its use in a particular scene.
The other method is to chain ideas starting with the comparison itself. In this case, we would start with the swan's neck, which might take us to giraffe necks, which might take us to elephant trunks, which might take us to steamer trunks, which might take us to steamer ships, which might take us to a graceful column of steam emitting from a chimney. Hmm. Maybe there's something in that image. Or maybe not. Maybe we need to start over again -- swan neck, to swan beak, to trumpets, to horns, to strings, to the long throat of a bass and the musician's hands moving up and down its length -- this might work. Physical love as a symphony has been used before, but maybe not this particular image in this particular way. Is it remote enough from existing cliches to work? (Side note: I don't ever want to see the word crescendo in a sex scene again.)
In any case, playing with these two techniques during revision will eventually lead you to a fresher idea than the one you had before. Once you've found your comparative language, the next step is examining it in the context of the scene. You want something that either fits in so smoothly that it won't jar the reader out of the context, or you want something so jarring that it requires explanation and behaves like a conceit. Generally, especially in genre fiction, choose the smooth over the jarring. These readers aren't there to groove on your philosophical constructs, but they do want a great, dynamic, engrossing story. Smooth, fresh language can help you deliver that to them.