Leadfingers: About how many yards do you have to wander off till it sounds good to the audience? ;-}

I think the key is just practice. I've always enjoyed singing harmony, and picked up the "art" of harmonic improvisation just by listening to lots of brilliant harmonists (both vocal and instrumental) and singing harmony to recordings--or even to melodies in my head as I showered or drove. (They told me to kill you all, but that's another matter.) Now I improvise vocal harmony lines for anything from folk to jazz off the cuff.