I got the large Gypsy Gripper for $5 in a box of stuff at a guild auction. It is FABULOUS. I use it on my long ruler and it makes me feel secure I won't cut my hand, and it also helps keep the ruler still. It sometimes prevents me from seeing the ruler markings though.
I am terrible about slding smaller rulers, but I don't want to pay for the smaller grippers. Honestly, I'm not sure how well they'd work- part of the stability seems to come from the space it covers.