My only suggestions would be:
Relax your shoulders down and do not lock your elbows straight or put lots of weight on your hands (which then push back on your shoulders). Think of your whole body as a fluid bending snake, and distribute the weight between your feet, saddle and hands. Feel how your spine flexes like a shock absorber in the middle. Try to keep from putting all your weight on your hands or on your butt.
If your elbows are locked straight and you are lazily heaping your torso weight onto your hands, then every time you hit bumps all that jarring is going to transfer directly to your shoulders. If you keep your elbows slightly bent (to absorb shocks) and keep your shoulders down and relaxed, then those bumps will be absorbed by the entire length of your arms and spine all the way to your legs as well. Adios jarring! Make your whole body into one long flexible spring.