Astels provides a good guide to test-driven development (TDD). The bulk of the book works through an example of developing a Swing-based GUI application using TDD. The book is quite Java-centric, but there's several sections at the end on unit testing tools for non-Java languages, and the general concepts should apply to any language. It was also nice to see how to apply TDD to a GUI, which is something I didn't understand before reading the book. If you're interested in trying out TDD, but you don't really know how to proceed, this is a good place to start.