The TalkingDrupal podcast had Alexey Korepov on to talk about Test Driven Development. Alexey has written the Test Helpers module, a development package that provides many useful utility tools for writing unit tests for your Drupal code.

When I work, I try to follow the principles of Test-Driven Development. I have found it to aid me in writing cleaner code, identifying odd coupling of components or crazy accidental dependencies between components. It also lets me write my API first by using mocks against interfaces I have defined. PhpStorm's auto-test functionality speeds up my test-driven development process.