If you’ve never written a unit test, write some. If you always write your tests after your main logic, write them before. If you always write them before, write them after. If all you ever do is unit testing, try integration and system tests, too. If you’ve never done QuickCheck-like property checking, do it. If you don’t know how to prove things about your code, write some proofs. If you don’t know how to represent important properties as types so that your computer will prove them for you, now’s the time to learn. If you don’t know how to design APIs so that certain classes of problems become impossible, start practicing.