I saw to put some rice in a ball or something and keep grabbing the rice for couple of tens of minutes every day. Didn't try it yet.
My doc explained to me that the joints - fingers or wrist - simply weren't made for this type of continued stress, building muscles hasn't anything to do with it, there's no prevention other than better/other handling of the gear... the main problem is that it not only hurts, but can get chronic if you ignore the problems for too long.
Btw if I just use the 60d+17-40L it feels light as a p&s, I suspect whoever created our skeletal layout didn't like heavy dslr gear :-p
Something like that
Hey, that looks useful, thanks! Did you try one of these grips, how does it handle in real life?
Edit: Does anyone know where to get a cheap China version of these grips? The US version from the image I found on ebay is $15 plus $90 shipping, doh.