As an example, the team I work on has been adding more precondition checking to all of our applications. The simple act of stepping back from the perceived data-flow and explicitly declaring what we believe should hold true has uncovered several bugs in our understanding of our applications.
We've likened it to having someone review a paper you've written: you often read what you think you wrote, not what's actually written.
This got me to questioning what others have found to be transformative in their development practices.
My second most important change was to learn how to use contract based programming, or, if the language has poor support, at least to use a ton of asserts. This, for me, feels like stabilising the code very quickly and, again, improved my bug/line ratio. It forces me to encode what I expect and the code will relentlessly and instantly tell me when my assumptions are wrong so I can go back and fix that before continuing to base the next steps on broken assumptions.