I've used a variety of mice and recently getting a lot of cramp from long hours editing in Photoshop. Please could someone recommend a mouse that will help.
Try a small one. Let your hand rest on the table and move the mouse by your thumb and nameless finger. Set the mouse fast enough and be sure to have an easy to use speed switch on the mouse.
Addendum: If you are a competent traditional pen user, then at least do as already suggested: Get Wacom. You get an enormous accuracy and speed boost for drawing in Photoshop. Beware cheap imitations. Their delay and unconsistency make you mad.
It's amazing how many designers do great work with just a regular mouse or touchpad. I prefer a trackball with scroll wheel and programmable buttons.
One main advantage of trackballs is you can hit the buttons without shifting the pointer at all.
You don't have to lift or move the mouse to operate it so it's much easier on your shoulder, wrist and hand.
Mine has 4 big programmable buttons, I've added copy, paste and scroll lock but you can assign any keyboard functions.
It's the Kensington Expert mouse, I highly recommend. You rotate ball with fingertips and left click with thumb.
Logitech has a good line of trackball mice, I've used many. Some you roll the ball with your thumb. Odd at first, I got used to it fast, but most other people won't be able to use your mouse, which can be an advantage.
I don't recommend handheld. Wireless is fine for a laptop and wired works with a desktop.
I'm using the Logitech M330 silent mouse, its a new model, ok not entirely silent but about 75% more silent when clicking than a regular mouse. It may not sound like such a big deal, but after using this for a while I wouldn't go back to a regular mouse any more.
It's also quite good for design work, nothing to complain about. Using the CS Suite daily with no problems. Not doing any intense Photoshop retouching however, just basic stuff mostly, for that I'm sure a Wacom is better.
Using one of those gel pads could help your cramping.