One word: Getatrackballmouse. OK, that's maybe 5 words, but it's still the best advice I can offer.
I like the Kensington Expert Mouse (surprise!surprise!) and, no, I don't hold stock in Kensington Microware Ltd.
But I've been clicking this thing for 9 years (well, I wore one out, but they last a long time) and I have no problems. When I have to work on a PC with an ordinary mouse, I suffer nearly immediate discomfort. I don't know how I ever used one of those things.
And I don't know why there isn't a huge class-action suit over the Microsoft Mouse

I mean, talk about "deep-pockets!