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Mouse usage is a big factor (and was the culprit for my tendonitis).  Learning the keyboard shortcuts for your most common operations means not having to jump to the mouse so often.  Remember the arrow and page up/down keys, too.

Unless you're hardcore, though, you won't get away from the mouse entirely, so consider its ergonomics too.  I found that a "shallow" mouse causes less wrist strain than the normal-height ones.  Compare the Apple Magic Mouse (shallow) to a typical Logitech mouse to see what I mean about height.  When forced to use a "high" mouse at work, I got some relief by attaching a small foam pad at the base.  I attached a Velcro strip to the mouse and the other Velcro piece to the pad so that I could experiment with placement.  (Removing the tape backing from the strip attached to the mouse later wasn't too much trouble.)

Another answer discusses the importance of the desk and monitor being at the right height.  Your chair is also an important part of that equation. Set a height that allows your arms to be high enough (straight or slightly down from your elbows, not up).  For shorter people, this might require a small footrest.  I find that locking the back makes a difference, so look for a chair that allows that (not all do).  There's a diagram in the [Mayo Clinic ergonomics article](https://www.mayoclinic.org/healthy-lifestyle/adult-health/in-depth/office-ergonomics/art-20046169) that shows the posture you're aiming for.  (One disclaimer: I've never used an office chair that had armrests that could go high enough for me.  I don't know if that's a quirk of my size or what.  So I can't speak from personal experience about the benefits of armrests.)

I've found that getting up and walking around frequently (a couple times an hour) helps with back issues.  This isn't always possible if you're in the zone or in the middle of a long game, but if you're at a point where you could pause for a couple minutes, and you haven't recently, try that.