Learning to use shortcut keys instead of the mouse. Stops you gripping the mouse all of the time, and actually speeds up your work (once you know how to use them). A simple example is
If I want to select all the text in a document I hold the CTRL key down on the keyboard and press the letter A on the keyboard once (rememberng of course to let them go :-) ).
If I want to only select a part of the document, I move the cursor using the keyboard, to the start of text I want and then hold the Shift key down and select the text I want by using the arrow keys on the keyboard, and then letting the shift key go.
On the assumption I want to copy it somewhere else, I then hold the CTRL key and press the letter C on the keyboard. That will copy the text you have selected.
move the cursor to the desired place in the document and hold the CTRL key and press the V key on the keyboard (Paste) and a copy of your text will appear at the desired place in the document.
Bear in mind, you can use those options for everything you do on the computer for copying and pasting, without even touching the mouse. I can tell you that it faster than using the mouse.
Using a trackerball. The big advantage is that you don't have to grip the mouse and move it about the desk. Just that in itself will eliminate the risk of getting tennis elbow.
When I used to teach IT, one of the biggest problems was with the way people held the mouse, or should I say "wrung the living daylights out of the mouse". Using the mouse in terms of grip, is a bit similar to gripping a tennis racket. If you grip it too tight and use it a lot, you will get tennis elbow. I used to do one day courses for people who were familiar with using the mouse and applications like Word, Excel etc and teach them all the shorcuts. Touch typists found it a blessing in disguise, as they no longer had to use the mouse, which slowed them down immensely. To this day it still stands.