To separate stacked, already-framed paintings in my studio, I use pre-slit foam insulating pipe covers found at my local hardware store. The pipe covers are inexpensive and provide the perfect amount of space between each painting to prevent scratches and dings. To use, I simply cut the tubing into three- to four-inch lengths and slip the slit side over the frame edge, one on either side of a piece.
I also use the pipe cover tubing when shipping my paintings to exhibitions. In this case, I cut tubing to fit each side of my work, and cover the entire circumference of the frame.
When building a new shipping crate, I glue a whole tube onto each of the four sides inside the crate (including the lid), then simply drop the painting into its cushy temporary home.