It may look funny, but attaching an Aimstick to your chest is a useful way to see and feel how the club shallows.
The Aimstick acts as a guide throughout your entire swing. On the backswing, it keeps your trail arm from going too far inside, a common mistake when “trying to shallow the club.” On the downswing, you can see the club get closer to the Aimstick from the corner of your eye.
As you start to rotate, the should start to relax and let the weight of the club do the work. This naturally rotates the club inside out for a strong, consistent strike.