Sometimes the back of a piece needs a lot of cushioning to bezel set a stone. There are many wonderful products on the market that will do this.
I personally like to use the low-tec version: Fun-tack on a bench block. It does, however, have drawbacks. It adheres very tightly to metal surfaces, with some brands having more adhesion power than others. Because of this, it is important to carefully lift the jewelry piece and Fun-tak together off the bench block, and only then peel the Fun-tack off the piece. Trying to peel the piece off the Fun tak on the bench block can result in the piece being bent out of shape.