I use Quilter's Select rulers, and I also use my hand to stabilize the ruler. I put my thumb and my pinkie at the left edge of the ruler, on the fabric. I splay the other three fingers on the ruler. I cut, moving away from my body, only until the rotary cutter reaches the highest point of my hand, meaning the farthest away from me. I don't ever go farther "north" than my hand. I stop, freeze the cutter - meaning don't lift it, don't move it, freeze it - and walk my hand, itsy-bitsy spider style, up the ruler. Stabilize the ruler with thumb and pinkie again, and repeat.