I was working on a new one tonight. The melody is there, but I feel it'd be better with a few slightly more ornate chords for once, so it's a question of playing around and finding out which chords containing the relevant notes sound best with the meaning of the words.
I don't think there are any rules I'm looking for about that. The only way is just try them out until they sound right. (And then I'll probably end up throwing out most of the ornate chords.)