I find trackballs where the hand rests on top and the ball is manipulated only with the thumb are much better than mice and trackpads for cursor movement. Our fine motor skills are more refined in the thumbs, not in the fingers or forearm, which is something video game controller vendors have known for years...
They've been making variations on the same thing since the early 90's - I have an ancient Mac ADB version in beige somewhere around here, which predated the invention of the scroll wheel...
i have the newer (blue) one and it has certainly helped. i have it positioned between my body and the keyboard, on the left, angled so that my arm is in a very comfortable, relaxed position when using it. it sounds odd, but works well.
it might be worth adding that even after many months of use i still don't feel as accurate with it as with a mouse (and a friend with one feels the same). but it is certainly "accurate enough" (i have a logitech mouse, configured to use the same usb receiver, but haven't felt the need to use it). if you do need very fine accuracy i think the best solution would probably be to combine one with a wacom tablet.
before that, i found that switching hands with the mouse helped. it doesn't take long to become proficient at left-hand mousing.
Logitech makes these - this is the newest version: http://www.logitech.com/en-roeu/mice-pointers/trackballs/dev...
They've been making variations on the same thing since the early 90's - I have an ancient Mac ADB version in beige somewhere around here, which predated the invention of the scroll wheel...