If you're working with Django, get a copy of this book and read it cover to cover. Django 1.1 Testing and Debugging is an excellent tour through the functionality available to test Django applications. It also covers in depth what debugging tools are available and how to use them.

One of the great chapters here is the one on understanding the Django debug page. There's so much detail on that page, it's easy to get overwhelmed and not be aware of some of the depth that is available.

The final chapter covers deploying with mod_wsgi, and includes details of how to load test your application on this architecture.

Don't be put off by the version number in the book title, this is still relevant to (at the time of writing) Django 1.2. You'll not only learn about testing and debugging features you didn't know were there - you'll also find yourself picking up bits of the core Django framework that you didn't know existed; I know I did!