As a not quite so old guy who is working on eventually developing carpel tunnel I find a Marble is WONDERFUL

 here are a couple cordless versions  

It takes a bit of getting used to and my kids hate it, but my hand doesn't go to sleep nearly as much as it does when I'm trying to use a mouse.

See http://www.logitech.com/index.cfm/products/productlist/US/EN,crid=2141