For a little clean up time saver after painting something with a rolloer and tray, simply place a small bag over your paint tray (such as you get from the grocery store) and when you are finished or you are changing paints, just pull the bag off, tie it up and throw it away.

Also, if you are using a brush, you can put a light coating of cooking spray or vaseline on the metal part of the brush so that the paint does not adhere to it and make clean up a bit easier.  Just do not get it on the brush itself.