This will sound strange but: Efficiently writing effective unit tests. Too often when starting out I would write tests that were redundant, dual purpose and overly complex. I would even test functionality that was not the responsibility of the unit under test. Now I focus on strong coverage of the API and refactoring each unit so that a high percentage of coverage is easy to accomplish while remaining meaningful.
This will sound strange but: Efficiently writing effective unit tests. Too often when starting out I would write tests that were redundant, dual purpose and overly complex. I would even test functionality that was not the responsibility of the unit under test. Now I focus on strong coverage of the API and refactoring each unit so that a high percentage of coverage is easy to accomplish while remaining meaningful.
This talk is one of many that helped me improve:
destroyallsoftware.com/talks/bound...
But mostly it has been learned through time and tears spent re-reading old unit tests that did not stop bugs and took way too long to write.