I use a small wireless optical mouse - Logitech M305 to be precise.
I have smallish hands so like a mouse that I can ‘hold’ with my arm of the desk, and click/roll with my middle pad of my index finger. Just as Geoff describes. I can get a pain in the middle of my hand, on the top of it, if I have to use a mouse that is too big. You’re supposed to hold your arm up in the air, but I can’t do that!
I also have painful finger joints, which is why I’m careful with mouse clicking etc.
It’s definitely worth looking at all sorts of different mice to work out what works best for you - Currys/PC World are good for letting you see how they feel. Not sitting at a desk, but it’s a start! You could ask a salesperson to let you use one of the mice on a computer.
I’d also pay Maplins a visit. They are not big and flashy with lots of marketing budget, but they really do know their business so I’d trust their advice. I’m not sure if they sell mice, though. Local Disability centres may be able to help you find somewhere that lets you try before your buy too.
If you use a computer then having a ‘bad’ mouse for you is a very painful thing!
Ironically, I’m off mouse shopping with my boyfriend tomorrow, but can’t give him any advice as he needs mice that are no good for me!