When I set poetry to music, I focus on the central images in the poem. To me, poetry is not words. It is the images that the words create. And thus, with the musical setting, it is important that the images in the poetry translate into musical imagery. Often, the accompaniment is the central means of creating the imagery. The world of the poem may be established within the opening measures of accompaniment. The vocal lines, while also participating in the musical imagery, have a primary function of conveying the words.
The song must have a cogent and dramatic form of its own. The form of the song may mirror the form of the poem. Or, it may transcend the poem. But, the music should stand on its own as a well-constructed entity. Thus, I always try to create the overall shape of the song before I start composing.