That's the same thing I work on. Recently, I've been trying to relax as much as I can and just make a stroke. If the top of the back feels nice and flat, the left shoulder goes up and the right shoulder goes down together.
Cheers!
Geoff Mangum
Putting Coach and Theorist
PuttingZone http://puttingzone.com
Golf's most advanced putting instruction -- you're either in the PuttingZone, or not.