Before you pour the paint into the tray, line the tray with aluminum foil. You can buy wider rolls of aluminum foil, so that it can completely cover your tray (depending on what size your tray is). This way, when you are finished, all you have to do is take off the aluminum foil and throw it away. Bam. Your paint tray cleanup is DONE! Make sure that the corners are pushed in and the edges are tightly pressed around the paint tray.
Who would have thought, right?