For large rulers - I recently got the ruler and cutter combo where the cutter is built in (Nancy's) or (Quilt in a Day) - but on other rulers, I use a spring hand clamp to clamp it to the table. Smaller rulers, I am still experimenting - have used the little felt pads from the dollar store (sold to keep doors from banging), and the green shelf liners. Small strips of shelf liner also help if you have a gap in an embroidery machine hoop.
Marysewfun