FOR THOSE WHO HAVE BOUGHT CARMONY: LEARN TO SING HARMONY WHILE YOU DRIVE:
This page has downloadable PDFs of lead sheets and lyrics to the songs Libby McLaren and Celia Ramsay used to demonstrate harmony singing on the Carmony album. They really recommend you learn these songs from the album by ear FIRST! Songs learned by ear are more easily retained, and your ability to sing harmony will improve faster.
You don't have your copy of Carmony, yet?