Imitating another writer is one way I get myself (would that be my ego?) to shut up. I get so busy concentrating on doing the assignment right (I've always been a whore for A's), that I forget all about content and usually end up surprising myself.
Or you can try this. List 10 concrete nouns. List 2 or 3 colors. Name a place. Write down a phrase you overheard someone say today. Write down a single image from a recent dream. Now put these on index cards (equal size bits of paper), one item per card, and shuffle. Now, use these -- whatever order they turned out in -- to write a poem.