-Stop using a mouse. Use a trackball or a trackpad or anything else. I have a death grip on mice and I bet others do too.
-If you have to use a mouse switch your mousing hand. Deal with the fumbling and inaccuracy. You'll figure it out.
-Lean farr back in your chair(legs propped up) and rest your keyboard in your lap. Shift position occasionally.
-Move your monitor so that you look in different directions. Laptops are great for this and that compensates for their other ergonomic problems. You can usually find a way to move them higher or lower and correspondingly shift the strain.