Let's get a few definitions out of the way: a simile compares one thing with something different (usually has the word "like" or "as" in it), while a metaphor is a direct comparison that isn't true on a literal level. So "the clouds hung low over the school, like dirty cotton balls in Masie's mop bucket" would be a simile, "the dirty cotton-ball clouds" would be a metaphor.
Similes and metaphors are a way to play with language and make your voice stand out; you just have to come up with your own. When we think of a way to describe something, the first thing that comes to mind is the same old thing that we've heard over and over again, maybe "strong as an ox," or "lightning fast." Those aren't bad, but they're not exactly original. Well, how to be original, then?
- Come up with your own. There is a trick to this, though: you have to have a certain sensitivity to what works. You can't compare things that are too similar (the cat was as soft as a kitten) or things that are too different (the cat howled like a dishwasher). The things have to make sense on some level, as in if you use a word for sound, then the comparison should fit that. The dishwasher thing fails on that level too, as the sound a dishwasher makes could maybe be described more accurately as a hum or a sloshing sound, maybe). Unexpected comparisons are great and make the best metaphors, but they have to make sense on some level. You can experiment with this, though. Think about what the reader expects and play around with some other options.
- Twist a worn metaphor or simile to make it your own. "Strong as an ox" might become "strong as a pitbull on steroids," maybe.
- Synesthesia is also a great way to come up with interesting stuff. It means using a different sensation to describe something else, like tasting a colour or hearing a sensation. "A burnt-velvet whisper," or "the cool, green tang of lime," for example.
Don't overdo it, though, or your prose might begin to sound unintentionally comical. For a fantastic book on the subject, check out Word Painting by Rebecca McCalahan.
Bonus: It's not really a metaphor or a simile, but zeugma is a useful instrument to have in your toolbox of descriptive writing. It means using one word to modify two others in a different way, like "She lost her watch, then her temper."
In todays 350 words, try to include one original metaphor or simile and use zeugma at least once.