Whichever Joe above said "if you can't, don't" is wrong! The way to learn to sing harmony is to keep doing it. Sing quietly enough not to bother your neighbors and friends, until you've got the harmony down on a song, but don't be afraid to softly experiment. I'm a pretty good harmonizer, but I love to try out new,sophisticated, and daring harmonies from time to time, and the only way to tell if my theories about new harmony lines will work is to try them out in practice. And you can't sing harmony alone. (Unless, of course, you're into Tuvan overtones, but that's another thread.)

--Charlie Baum