I have to say, after dozens of productions and days that seemed never to end with the same basic rig, that I agree with the pads-don't-help school of thought. Most hand-held aids just infringe on the flexibility of movement and run counter to the reason that you are hand-holding in the first place. I agree with the deep massage, and regular chiropracter adjustments. Learning some Yoga helps a lot for rebalencing your body as well. Also, practice and strength training (legs, abs, back, shoulder muscles), just as with any athletic persuit, go a long way.
.Just a couple of small hints: wearing a skin tight 'Underarmor' undershirt under your shirt, ends abrasion of your shoulder and wicks away sweat. Reduce the camera's weight using a lithium ion battery like Dionics - that can remove 7 big pounds or so. Harass the sound department to use newer and lighter receivers, or put them in a pouch on your waist (a bit more trouble to remember to unplug, but you lose up to 5 pounds). Over the hours, a few pounds less can really help.