For Windows users with desktop keyboards that have numeric pads, I also suggest turning on the Mouse Keys feature. One thing I always found most stressful with a mouse was dragging with the button down and trying to make precise movements. I felt like I was cramping my hand every time I did that. Mouse Keys makes precise mouse movements easy:
https://support.microsoft.com/en-us/help/14204/windows-7-use...
Or on a laptop keyboard - particularly on a ThinkPad - use my JKLmouse program (an AutoHotkey script) that does much the same thing without a number pad. Whenever you hold any mouse (TrackPoint) button or the Caps Lock key down, various keyboard keys turn into mouse movement keys. It works on any keyboard but is especially nice on a ThinkPad where you can press a TrackPoint button, use the TrackPoint to get the cursor about where you want it, and then - with your hand still on the home row! - move pixel by pixel with the keyboard.
I don't know of macOS or Linux equivalents for these, but would be interested in hearing about them if anyone is familiar.
Another RSI tip concerning mouse usage: try to learn using the mouse with your secondary hand. Initially it's very hard, but after few weeks you can do all but the most precise things with the secondary hand. Changing hands can significantly reduce the pain. I have two mice (that may look weird but I find it useful): an ergonomic one for the right hand, and a symmetric trackball for the left hand.