Two things:
I use Steady Betty ruler strips. They're little pieces of foam and they grip really, really well.
Second, I do the same as Ghostrider. Put your left hand on the ruler and with your right hand, start cutting at the bottom of the ruler, moving up and away from your body. When your hands are even, stop cutting. Do not lift the cutter, just hold it there, and walk your hand up the ruler, keeping pressure on it. Do the itsy-bitsy spider thing. Repeat the process until your cut is finished. The trick is to not have your cutter hand pass up your ruler hand.
Hope this helps!