Went to MQX in Providence RI last week and took a class from Dustin Farrell on Radical Rulers with the Long Arm. As he was whipping along someone asked about the ruler slipping. His response kind of blew me away.
He laughed - and commented it was nothing that a little "old fashion" Rubber Cement would not solve - he continued to say that all you need to do is spread a little rubber cement on the back of the ruler and let it dry completely.
The up side of this technique is that it is cheap, easy to see through, rubs off the ruler easily and will not harm the fabric.
I tried this today on one of my rulers and it worked fantastically!! Best I can see is that it is a win win situation. I have tried the clear rubber stick ones (which were thick, the ruler did not grip where I need it to continuously along the edge because it was not flat and and only worked sometimes) , the sand paper ones (which blocked the visual on the seams) and others - but this is so easy, the ruler stayed flat and I could see through it.
Try it you will be amazed!