I consider myself a pretty adept "instinctive" harmony singer. High harmonies are fairly easy, and I don't give them much thought- I sing what feels like should be there, and if the harmonic doesn't ring, I change as needed. I believe the secret of a really resonant harmony are the notes that depart from the predictable path of the harmony, giving it a sense of surprise and distinctness. Some singers are also strong melody line singers, and some aren't, and the ones that aren't will drive you up the wall when you do harmony.It also takes a lot of practice to keep your "harmony ear" in tune, and I often sing harmony in the car with tapes.