I always have students read the whole poem through once and then read it again and draw lines through the lines to show where there are shifts in thought. This helps students see the poem in smaller pieces -- those pieces usually end up being different/separate images, metaphors, symbols etc. With consideration of how each of those distinct elements works, they can then put the poem back together and see how the elements work together to create the meaning of the poem.
Finding the controlling metaphor--the tension in the poem between two unlike things or ideas--is the key to understanding a poem. Hopefully in the free verse poem, the metaphor is not too abstruse; then, the students should with the aids suggested above--sound, and rhythm--be able to realize what the poem is really about as they recognize the surface meaning.
It's been awhile since I taught middle school, but the basic principles are the same for nearly every audience, it seems to me. I always like to start with sound. If they've read it on their own first, chances are good they did not read it aloud. If they're seeing it for the first time, adding the element of sound will have more impact. There should be a natural rhythm and flow to a free-verse poetry, something which generally resonates with young people to whom music and lyrics are so important. As mentioned above, follow that with a discussion of imagery and other literary elements, then read it (or have one of them read it) again. An effective poem generally "blossoms" for readers the more they understand and feel what the poem is trying to express.
For free-verse poetry, as with all poetry, students should look for theme or purpose. After this is decided, then they might look at several of the following--the figurative language and how it relates to meaning or effect; sound devices such as alliteration and assonance and repetition and how they emphasize key words, tone and how tone is created and whether or not there are shifts in tone, the speaker and the situation.
I found my 10th graders are quite good at expressing their appreciation and understanding of a poem by creating a slide show of lines of the poem complete with images and music that convey tone and meaning. I find that they work well in this medium, and their grasp of the poem is easily assessed.