On the Zend Developer Zone there's a new article from Keith Casey looking at some of the "best practices" when it comes to writing unit tests for your applications.
If we are starting from absolutely nothing, building tests as we go is a near-trivial process. [...] Unfortunately, most of us don't have this situation. We walk into a project with documentation ranging from non-existent to seemingly-useful-but-really-out-of-date, and hundreds of thousands of lines of code, and the coding standards and practices of every developer that has come before us. In this situations, the above Unit Test example is almost mocking us. We can't start that simply! Or can we?
He walks through some of the steps he and another project lead took to start their trip into testing - finding the most problematic bits of code first (based on bugs in the tracker) and testing them, figuring out how to test rapidly changing code, finding common code used in several places and testing that and