I use my right hand for the mouse and have had some small problems with it, but my left hand is the one that usually gives me pain. This is because I always have it in possition to press and hold the left Alt button to move around in Maya, do Ctrl D for duplicating since I do that quite a bit, and to do Ctrl + undo as well. It stays in that position permanently when I'm modeling in Maya. As said before, I also have a habit of putting a death grip on my mouse, which really hurts in the long run. On top of all that, I'm probably in one of the worse sitting possitions of all to be in front of a computer. I have my couch budded up to my computer desk. Believe it or not, it serves as my bed, too and everything is in reach from it. My computers, my game systems, my TV, my sterio, my keyboard, etc. Yep, I'm lazy.