use.perl.org reader amoore’s first journal entry, How to Sneak Testing into Your Development Team is an excellent personal retrospective of improving the quality and test coverage of an existing, large, untested application. I particularly like his (her?) second and third suggestions — especially racheting up quality.
Not every language or environment has the same tools as Perl (particularly the cannot-say-enough-good-things-about-it Perl::Critic), but these suggestions can work with minor modifications in many environments.
Hey chromatic - I'm glad you noticed my post. The idea for it when you stood up at YAPC::NA 2006 in Chicago and asked a question. I think it was during Andy Lester's talk on Technical Debt. You asked something along the lines of "Assuming that I've drunk the Kool-Aid of testing, but none of my coworkers have, what can I do to get started and how to I encourage them to follow along?" (excuse the paraphrasing, it's been several months.)
Since I was in the process of doing exactly what you were asking about, I thought I could describe how I was (successfully) doing it. My testing environment and my teammates' acceptance of it have come a long way since then, so I encourage others to follow my recommendations.
-Andy