During the Testing Tools tutorial that Titus and I gave at PyCon, there was a short discussion on testability -- what makes software more testable? I mentioned a list put together by Michael Bolton, and summarized/enhanced by Adam Goucher in this blog post. Recommended reading, both for developers who want to add testing hooks into their software, and for testers who want to know what to ask for from developers so that their life gets easier (and if you're one of the unfortunate souls who have to deal with Java or .NET, this blog post by Roy Osherove talks about testability and pure OOP.)
Although our tutorial was focused on tools and techniques for implementing test automation, we also mentioned that you will never be able to get rid of manual testing. Even though the Google testing team says that 'Life is too short for manual testing' (and I couldn't agree more with them), they hasten to qualify this slogan by adding that automated testing frees you up to do more meaningful exploratory testing.
My experience as a tester shows that the nastiest bugs are often discovered by manual testing. But when you do discover them manually, the best strategy is to write automated tests for them, so that you'll check your application in that particular area from that moment on, via an automated test suite which runs in your continuous integration system.
You do have an automated test suite, right? And it does run periodically (daily or upon on every check-in) in a continuous integration system, right? And you have everything set up so that you're notified by email or RSS feeds when something fails, right? And you fix failures quickly so that everything turns back to green, because you know that too much red, too often, leads to broken windows and bit rot, right?
If you answered No to any of these questions, then you are not testing your application, period (but you already knew this if you took our tutorial -- it was on the last slide :-)