1 Cron. 29:14-15
"But who am I, and who are my people, that we could give anything to you? Everything we have has come from you; we give only what you first gave us! We are here for only a moment, visitors and strangers in the land as our ancestors were before us. Our days on earth are like a passing shadow, gone so soon without a trace."

I think that if you can fully subscribe to the idea that God gave you everything you own, it is alot easier to let it go.