>>A guitar-buddy of mine in WV recommended just tooling around with preexisting songs that you like -- rearranging them, inverting the chords or the melody, changing the time signature. <<
Busted! "Ink and Pen" started out as the tune to the Beatles' "A Day in the Life," to which I originally put "I bought a pen today, oh, boy." Morphed into "I brought a brand-new pen today;" then I changed the melody and then the structure of the following lines so that the underpinnings were completely gone. Sort of like making papier-mache over a balloon and then popping the balloon once everything has dried and set. (Or inking and painting over a pencil sketch and erasing the pencil lines, or knocking down the scaffolding from a finished building).
Songs usually come to me via one phrase whose vowels and rhythm practically dictate the melody. Then comes the rest of the chorus, whose melody is usually fully formed before I even pick up a guitar. Lately, though, I have begun to visualize the chords even as I form the melody. Most of the time, I tell myself I must write a certain song, only to have something else worm its way into my consciousness and demand "Write me now, dammit!"
I am always open to new tricks to shake things up, and to unusual choices of chords to affect the harmony of the melody. But every note must serve the lyrics, and every lyric must be singable and flow with the song. One of my pet peeves is a gorgeous poetic lyric and then a clunky prosaic phrase that just thuds and lies there like a dead fish thrown onto a boat deck.
And no matter how much I may like the finished product, I'm always open to suggestions on how to refine it. There is a big difference between believing in your songs and being married to them.