It ain't so much about where you hold your marker, though the shoulder will be more stable if done correctly. The most important thing todo is use your body as a shock absorber. Walk around at about two thirds of your normal height, if you are 6 feet tall, have your head at 4 feet. This way you have a "hinge" in your body, which will enable you to walk or run whilst keeping your upper body relatively still.
As usual, this will require practice.