I think in a new, happy relationship it's the "Absence makes the heart grow fonder" phrase but in older (maybe not so fun) relationships it's probably "out of sight out of mind"
"Out of sight, out of mind" is the right approach for a relationship that just ended, and "absense makes the heart grow fonder" is applicable to short periods of separation between two loved ones/good friends.
Absence makes the green eyed monster come out to play.
Absence also seems to have a directly proportional effect (affect? I never know...) on my phonebill. He must just flippen come home now or I'm gonna have to start selling organs on the black market. I have a particularly healthy spleen should anybody be interested....