For my larger rulers, I use a hand-grip (spring action) and clamp one end of the ruler to the cutting board and then just apply pressure to the upper 1/3 while I am cutting, and it works like a charm, no slipping. For my 24 inch ruler and thicker fabric, I have used a clamp on both ends to hold the ruler taut and they hold firm without problems. I got them at Harbor Freight for a couple dollars each.
Marysewfun