I’m currently rewriting dwidder [1] and I just spent way to much time on a unit test for the registration view. These are the things that made me stumble:

1. Write fixtures. Unit tests run on an empty database. Somewhere deep in your code might be a reference to one of the default applications, like django.contrib.sites, which will fail, because there is no default site object in your database.
2. Don’t subclass unittest.TestCase. Use Django’s TestCase class instead. Otherwise you will miss all the fancy stuff, like automatic fixture loading.
3. Import mail, not outbox. Django provides a dummy email outbox. But to make it work as expected, you have to import the mail module. If you import the outbox directly, it will always be empty.

[1] a now defunct site where you could tweet by sending an SMS to a German mobile phone number