Somewhere on this board I saw the suggestion to coat the backs of rulers with rubber cement (sold in office supply stores). It works great for me. It takes quite awhile to dry if the humidity is high, so don't do it on a day when you're in a hurry to use your ruler. The back of your cutting mat can also be coated with it if you have a problem with it slipping. It doesn't seem to hurt anything I've tried it on, and it can be peeled off and redone later if it gets too ugly from picking up fuzz.