Use this lat stretch to eliminate the need to bend your arms in the backswing and try this over the head hip and torso stretch to increase the separation between your hips and torso to make a more powerful swing
Be like the Duke Lady Blue Devils golf team and play this jump back drill with your friends, wherever your putt ends up you have to jump the ball back the length of your putter and sink the following putt from that distance
A common mistake on the backswing is to hinge instead of cock your wrists, a perfect drill to help fix this is to pre set your backswing by cocking your wrists right up in front of your face at address and then turning into a full backswing