If you have issues with arthritis in your hands or wrist, I reccommend the Martelli rotary cutter. I find it easier to grip and control. When I use that one I am much less apt to stray on the cutting side. I tip the cutter ever so slightly toward the ruler and that helps to keep me from flying off the wrong way!.
I wind up holding the ruler down with my whole arm to keep it from wobbling around - which I know is not the smartest or most efficient. I saw a ruler here the other day which I can't remember the name of but it had a wodden handle on the ruler. That looked like it might help me hold the ruler straight!