It’s no secret that the best putters in the world keep their lower bodies as still as possible, employing a rocking motion of some sort with the arms and shoulders. Here’s a quick drill you can do anywhere to check if you’re keeping your lower body still.
Take a belt and wrap it around your legs just above knee level. There’s no need to go overly tight here—just secure it enough so it doesn’t slide down. Next, take a necklace with a weighty pendant and hang it from the center of the belt.
With the belt, necklace and weight, you’ve created a pendulum. As you make your putting stroke, the pendulum should stay stable. If your legs sway, slide or if you turn your lower body—even just a tiny bit—the pendulum will sway as well.
Practice first making some putting strokes without a ball to concentrate squarely on keeping your lower body stable. Once you get the hang of it, start rolling some putts and see if you’re still able to keep the pendulum motionless. With a little practice you’ll quickly see how a stable lower body makes it easier to hit your putts more solidly and with better distance control.