I get hand pain and numbness after about a 15 mile ride on my flat bar mountain bike. A looser grip helps some. Doing my weight work also helps. Wrist curls make my hands feel better right away, and there are a few other exercises that help loosen things up. The numbness is usually worse in my right hand, and makes computer games really unpleasant. Knitting and handspinning seem to mitigate the problem, provided I use good technique.

This is great for my craft productivity and rather awkward otherwise. It would be worse, but I'm one of those right hand for the mouse lefties, so most of my daily activities are not impaired.

I've been test riding bikes to see if different bars help. So far, a narrow cruiser bar seems best for my hands.