This is going to sound like an infomercial, but -- Clear Flex-Seal. Yes, that spray-on rubber stuff that you've seen ads for a bazillion times. Make sure it's the clear and not the opaque variety! Just give your rulers a light misting on the back and let them dry. It does give them a bit of a frosty look, but nothing that interferes with using the ruler. I have tried invisigrip, sandpaper dots, tape, rubber cement, but nothing else works as well as the Flex-Seal.