Once I get my ruler exactly where I want it I put a weight on the end furthest from me, just to help in keeping it in place as I tend to unintentionally loosen up on the ruler while cutting. But I also walk down the ruler as I cut as someone else described. Cut some, move my hand up the ruler, cut more, keep going till I'm done.
My "weight" is really just the head of a smallish sledge hammer that I made a fabric cover for so you can't tell what it is(and contrary to his "I'm gonna find a handle for it soon" speech), DH hasn't even noticed it's not in the garage anymore. I use it all the time for so many things while sewing and cutting. I also "borrowed" a 2" socket from the garage(he keeps all his real tools at work anyways) and turned it into a weight/pin cushion.