What do you mean by "cliche" exactly?
A cliche is a word or phrase so commonly used as to make it instantly recognizable. Common cliches include: "to run like the wind", "as fast as lightning", "death's doorstep", "bat out of hell", "scared to death", "badge of honor", "as black as coal", "loved and lost," etc.

Why shouldn't I use cliches? They're easy and everybody knows them already.
Precisely. Everybody knows them already. No one wants to read the same thing over and over. They want fresh and original content, unique perspectives, varied angles. Reading a poem filled with cliches is boring. You may think you're helping the reader by providing them with something they know, but in fact you are simply advertising that you are too lazy to think of your own descriptions and phrases. Harsh, perhaps, but true. People don't read poetry to be reminded of what they know. They want something new, different, unique.

So how do I avoid using such common words and phrases?
I'm glad you asked. Below are some tips to help keep those pesky cliches out of your poem and back in TV sitcoms where they belong.

Make a List
One way to prevent cliche usage is to make a list. Write the object you're describing at the top of the page (for example: love). Then underneath it, list at least ten words (adjectives, nouns, whatever) or phrases (sentences) that immediately come to mind in association with that element.
For example: Love
Words: red, rose, lust, passion, reject/rejection, marriage, family, Valentine's Day
Phrases: to fall in love, to have loved and lost, blind love, all's fair in love and war, love conquers

OK, now you know what to avoid. Definitely avoid the phrases, DO NOT use them under ANY circumstances. As for the words, you can use them, just be careful. Use them in different, even surprising ways. Try describing a noun using an adjective that would usually describe its opposite. Place pieces of paper with nouns written on them in one bag, and paper with adjectives written on them in another. Draw from each, put them together, and play with the results!

Use a Thesaurus
This means don't use common, everyday (read: boring) description words like very, good, interesting, bad, etc. Don't be afraid to use other words, even new ones. Of course, never use a word if you don't know its meaning, but take this as an opportunity to add to your vocabulary and learn some fun new words. Just be careful not to get too esoteric with your words- make sure they match the rhythm, style and formatting of the rest of the poem. Large obscure words can cause readers to stumble and break their concentration. Make sure it fits.

Check it on Google
So you think you may be in possession of a cliche, eh? Type the suspected cliche into the Google (or Yahoo! or any search engine) search bar in quotation marks (example: "run like the wind"). See how many results come up (for the example: 411,000). If it's a really big number, then it's a cliche. If it's a smaller number, say, in the low hundreds, then it's probably fine in your poem. Extra points if the search results are only in the double digits.

Use it for Irony/Satire
**DISCLAIMER!!** This can be a difficult task to pull off, and must be done very carefully.
Yes, yes, earlier you were told NEVER to use cliches under ANY circumstances. It's still true, you really should "avoid them like the plague" (groan). BUT for those of you who believe that no rule is absolute, that you can, nay, must do what others say you shouldn't, then read on (with caution).
It's important to remember that if such irony and/or satire is done poorly readers will not "get" the irony, and instead think that yet another lazy poet didn't care enough to produce something original. The best way to avoid this is to ask others to read it (without too much explanation beforehand) to see if they can intuit your true satirical/ironical intentions. If so, great, you've got yourself an interesting and potentially witty poem. If not, then it's back to the drawing board (writer's desk?) where you'll either decide to scrap the idea (not necessarily a defeat- you'll know if your poem really needs it or not) or re-work it until it shines.