Phil Collins has an amazing ear for picking out great and unusual harmony lines. He did it with Genesis for years, both behind Peter Gabriel and then just multitracking himself, then solo and with other people. "Easy Lover" has one of my favorite PC harmony parts. It's nuts, and Phillip Bailey also has an incredible voice and does some great harmonies on this song. It's a true duet.
This might sound kinda lame, but something that my wife and I do is when we're driving around in the car, we always have tunes on and we sing along in harmony. She sings the melody, and I pick out the harmony. I sing tenor in the church choir, and of course background vocals in countless bands, so for me it's actually more natural than singing lead. Try singing Phillip Bailey's high harmony (falsetto if necessary, which it is for most males) in "Easy Lover". It's totally insane the way it jumps around.