Jay knows not of what he speaks. As long as I shave my hands before riding, I have no problems. :scream:
The only time I have a problem with my hands going numb is about 15 minutes into a white knuckle trail ride / enduro. Once I hit the first reset and rest for a few minutes, it goes away for the rest of the day, but pushing too hard, going too fast, and gripping too hard at the beginning of a ride cause my hands to go numb.
Rubber mounted bars, silicone sealant in the ends of the bars, and proper sized grips also help. I used to use the big oversized ProGrips because they were nice and soft, but I had less problems when I went back to normal sized grips. I have bigger than average hands, but the ProGrips were too big.