Try to keep the mouse on the table as near your bellybutton as possible. So the mouse between you and the keyboard at about 30-45 degrees. It takes a little while until you get used to move the cursor properly, but it pays.
I have used a mouse or a digital tablet more than 20 years and I had much trouble with my cold fingers and aching shoulder, but after I changed the mouse near my stomack, these symptoms eased up.