I wrote this as a comment on a post on Don Share's blog:
I first started writing poems regularly around 1983 or so, in a free-verse mode under the influence of Denise Levertov (CW teacher at Stanford). And I started writing songs regularly around 1985. For a long time, the two modes were quite separate for me: songs were in rhyme and meter, and poems weren't. In the mid-nineties, though, the boundary began to blur a bit, as I began to write more and more poems in meter (rhymed and unrhymed). For me, now, the only identifiable difference between verse written to be a song and verse that is not going to become a song is simple: if the verses contain a lot of enjambment, it's hard to make them a singable song, because it's hard to write melodies that won't ignore the lineation. But if the verses don't have a lot of enjambment, then they can easily be made into a song, if I find a good melody and chords.