I bought a small roll of "Clear Grip" by Unique. As it comes in a large piece, you just cut it a tad smaller than your ruler and it then friction fits on the back of the ruler. It works really well. I've done 2 of my rulers and still have some left for other things.

I too, use the crabbing method on my 6x24. (oh but it is still possible to slice off the end of your finger nail if they are long....don't ask... blasted thumb.) And yes, the bumping of the ruler IS a tad annoying.