hmmmm...I learned compersion by being dragged kicking and screaming into accepting it. I had to do a lot of self reflection, introspection and communication to get there. I still fail at it sometimes, but I know the "why" or my stumbles. For me, so far, it was like a boxing match between jealousy and compersion with me as the ref...
My compersion isn't related to poly however, as the person I feel compersion for is not involved with me...I have learned to be joyful for her when she is happy or with others.