We use plastic squares. On the back of each squares we double stick tape a tape a thin piece of clear polycarbonate that extends out from the tab under the print. This is helpful as there is now way for the paper to slip under the tab.
Andy is right in that it helps to have a durable tab. I did this one edition once and didn't notice that my take-off was slowly sawing down my tab. It made it really hard to register later colors.