Here's a practical list to help learn harmony singing.
Lorraine Hammond taught us this in a wonderful workshop at "The Woods Music and Dance Camp" in the mid 90's. I've referred to it since, and have used it to teach basic harmony singing. For information about the Woods Camp, held in Ontario, Canada every summer click here!
LORRAINE HAMMOND'S HINTS FOR VOCAL HARMONY:
MELODIC INTERVALS FROM FAMILIAR SONGS
(major except where specified)
Second: DO A deer
ARE YOU sleeping?
Minor Third: A-LAS, my love
Third: KUM - BA -ya
IF I loved you
Fourth: HERE COMES the bride
Fifth: TWINKLE TWINKLE little star
DON'T THROW bouquets at me
Sixth: MY BON-nie lives over the ocean
BE-YOND the blue horizon
Seventh: MA-RI -a
All at ONCE AM I several stories high
Second: THREE BLIND mice
Minor third: THIS OLD man, he played one
Third: Down in the VAL-LEY
Fourth: BORN FREE
Fifth: AS TIME goes by
I WISH I was in the land of cotton
Sixth: BYE -BYE blues