I don't write a sing regularly, When I do, I explore everything I can about the subject in the way I would write an article or essay. Then I let certain words dictate was sings and work for fresh rhymes. I try fo interior rhymes, irregular stanzas to get away from the sing-song iambic pentameter or other usual models. I work for specificity and attempt not to employ footnotes allowing the song to speak for itself. I attempt to follow the Guthrie model saying a lot in a few words (like Hammingway). I try to very choruses from verses in music and mood. I work for consistency in the voice. If it's an "I" song, I'll stick with that. I try to visualize for whom the song is to be sung.

If you really want to get into it, Sheila Davis's books, especially her Workbook is the most helpful I've found although the orientation is toward writing commercial songs. The other useful books are Jim Webb's book and Molly Leikin's "How To Write A Hit Song".