This great little slideshow, Teaching Poetry, at Slideshare, illustrates many of the types of poems taught by Kenneth Koch in Wishes, Lies, and Dreams, my all-time favorite book on how to teach poetry.
You can analyze each slide/poem and then have your students write something similar. It is amazing how rapidly they come to understand what metaphoric language is and how to use it. I have also put some more specific directions and other examples of poetry on my Website at Quicksteps to Teaching Poetry. This was one of the favorite parts of my Liberal Arts Teachers class in English.
A related site, Poetry Tools, offers a nifty little Flash tool so that students can generate some nice little Metaphors right online, or download the tool for desktop use. This is a nice way to introduce the poetry writing sessions suggested by the Teaching Poetry slideshow.
And be sure to get Koch's book if you want to know more. It's available at Amazon.com.