If you're cutting away from yourself, you probably have a tendency to lean out away from the ruler.
Try "walking" your hand up the ruler, and you do it like this: Put your hand on the ruler (I usually place my ring or pinkie finger half on the edge of the ruler and half on the fabric). Push the rotary cutter up until it's even with your hand, and stop. DO NOT MOVE THE CUTTER. Hold it steady right there. Now on the hand that is holding the ruler - lift up the thumb only, not the other 4 fingers, and move it up right under your 4 fingers, and set it down. Once it is set in place, then move the other 4 fingers up the ruler. Think of it as playing itsy bitsy spider up your ruler. Once your ruler hand is set, then continue cutting with your rotary cutter until it is parallel with your ruler hand again.
It sounds complicated, but once you do it a few times, it gets easier and faster.