Originally Posted by Prism99
Aside from putting grips on the bottom of the ruler, it's standard to keep half your hand off the ruler. This steadies the ruler because your hand is not likely to slip against the fabric.
I've found this to be very helpful in cutting accurately, too. I have invisagrip on my rulers, I have at least my little finger off on the left side, and I move my hand up as I cut. I get way less sliding with this.