They make your writing flat, uninteresting and impersonal, and yet come oh so easily. I feel like there are all kinds of cliches all around us every day. Avoiding them in our own work is what truly gives us our voice and makes us unique. There are some things I think can help us do just that.
Detail. I firmly believe detail is what makes the writer and grounds the reader in any story. Without detail the story means nothing. Here's what I mean. Compare:
Blake wanted her so bad it hurtwith
Blake ran over and over in his mind the lopsided dimples that deepened when she saw him, the way she shuffled through radio stations like a deck of cards and stood on her tip-toes at drinking fountains, and the merest whiff of the cinnamon vanilla scent of her hair put a constrictive pressure all around his ribcage.Which is more interesting?
Research and experience. Know a lot (from books and personal experience) and get realistic in how you describe things. I once heard somewhere that some famous writer spent an hour looking at the faucet dripping so he could describe it for himself and not have to rely on anything trite and overused. Maybe we don't have to go that to those lengths, but you can bet that writers description of a faucet dripping was absolutely not cliche. Experience and read about everything you can. That way you can know for yourself and know what and how other people have written.
Be honest. No matter how painful it his, dig down until you get to the real gut emotional truth of something. Nothing genuinely true is cliche. Be honest with your characters, your story, and yourself. There are lots of ways to do this, too. Some writer pour their whole heart into something, whereas writers like Ernest Hemingway keep things incredibly gritty, stark and factual and let what they present do its own emotion invoking in the reader. Whatever works for you, but be honest.
Even being conscious of cliche helps you avoid it. Try the best you can, your writing will benefit greatly from it. Hope this helps, and happy writing!