Two things.

1. And the most important one. My guitar teacher taught me that. Whenever I feel pain in my wrists, hands or fingers I stop, no matter what. I take a break, see how it goes afterwards and if it’s not better I stop doing what I did for the rest of the day. Of course sometimes it’s important work stuff that needs to be done. As for me, most of the times it’s my right hand that hurts from mouse useage, I switch sides and get urgent things done, but still stop afterwards.
2. Sports, as usually, building up muscles in my hands and arms helps me personally. Not sure if there is scientific evidence for that.

But yea, number 1 is the most important thing, no job is worth having pain for the rest of your life.

Additionally I like roundtrip tests.

For example we have two data formats and support conversion between both of them.

So I have tests that convert from A to B and back to A. Then I can go and call assertEquals on them.

It’s a very cheap test, that tests all functionality of the conversion itself.