In my last class, the gal showed us a clear quilting tape (has kind of a blue tint to it) that you can just use on the bottom of the ruler. You will feel the difference immediately because the tape causes a bit of friction. It is much easier to control the ruler with the tape but you still have to apply pressure.
Another thing that this teacher pointed out is to NOT plan to hold the ruler in the middle. She holds the ruler on the first third and cuts the first 1/3rd then moves her hand up to the middle cutting some more, then moving the hand up to the end and finishing the cut. It really helps to control JUST the part you are cutting.