For me, at least, they're a part of writing that are difficult to practise. I've read to never use a simile you've seen in writing, and it seems like reasonable advice to me. If it's that unique a simile, you're just copying someone outright; if it's not, why bother using it? As such, my only choice seems to be either trying to change another writer's exact wording—which doesn't seem right—or coming up with new ones outright.
The process seems to be
- Find something that could be compared to something else without it obstructing the flow of the story.
- Figure out something interesting with the quality to be emphasised.
- Make sure the compared thing doesn't conflict with the narrator's knowledge or setting (e.g. a comparison to something that doesn't exist in the time period).
That's about how it goes with me. Recently, I had the idea to start a "Simile Bank" on the phone I carry around, so that when a good one hits me out in the field, I can save it right then. Unfortunately, it's rather rare that that happens, but it's a better bet than trying to puzzle it out in the midst of writing. I have about four right now, but you'll have to wait until they make it into my stories before you can read them. If I posted them here, after all, somebody else might put it into writing, and then I can't use it.
How are you with similes and metaphors? Other figurative language? What methods do you use to keep your comparisons fresh?