Be sure to cut with your rotary cutter straight up and down, not leaning in at an angle and pushing on the ruler. If I can, I keep my little finger off the edge of the ruler to give it some stability. And moving your hand along the ruler is good advise. If you think of it like this- if you don't move your hand, it ends up becoming a pivot point, just like if you were using a compass. If you move it along, your hand stays next to the cutting and there is a smaller chance your ruler will move. Also, when you do that, leave the blade IN the fabric, slowly move your hand, being careful not to jostle the ruler and put it down carefully, again so y don't jostle the ruler. I also like invisigrip.