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If you already know about JUnit and TDD [Test-driven Development] and you're not using it, you should be waking up in the night in a cold sweat, because you're being less than professional.

He's right: test-driven development is enormously liberating. But there are two sticking points that exist primarily because you don't get the benefits of TDD unless you have very high test coverage. First, it is virtually impossible to retrofit tests onto an existing project, even for new code in that project. Second, TDD is obviously suited for cases where the input and output are easy to specify, but what about all that code involved in producing the user interface? Then again, this second concern might be obviated by better modularization. Which is of course again something you'll need to build into your project from the very beginning.