I've not yet tried it, but in Sparky Imeson's "Mountain Flying Bible" he suggests trying the following:
"When turbulence causes fishtailing, step on the right rudder (one quarter of its travel) and apply left aileron to maintain wings level flight."
He writes that you may need to experiment with rudder pressure. The technique allegedly dampens yaw oscillations. The other thing to do in penetrating turbulence is to slow down.
I would recommend Imeson's book even if you don't plan to do any mountain flying.