If you are having trouble with your hand moving while cutting, try the Tru Cut rulers and their ergonomic rotary blade cutter MY COMFORT CUTTER Http//:
www.graceframe.com. The rulers have a raised lip and the cutter has an area that rides on that lip to get straight cuts every time. I also purchased their Tru Grip rubberized pads to put on my rulers so the ruler won't slip around as I move my hand. You have to pick up the rulers instead of sliding them but the straight cutting is so worth the effort. Look at their online catalog for the rulers it has great pictures of how they function.
I just bought a second 24" ruler and Comfort Cutter and had them shipped to my daughter's in CA. I want to leave them there to cut with when I go to visit. Because you know we'll be quilting while I'm there!!!