Along with holding firmly I spread my fingers out across the ruler and is possible place my pinky on the fabric snug up against the the ruler. This also keeps it from slipping.
Originally Posted by
QuiltedCritterLady
For me, the trick is pressing down with the ruler. You have to have a firm hold on it when you cut. There are also some little rubber things you can get that attach to your rulers to keep them from sliding around on your fabric. I got mine at JoAnn's, but other places probably have them too.