Testable Java
It's often hard to retrofit unit tests in code that wasn't designed to be testable. In this paper, Michael Feathers describes a simple rule that you can use during development to assess the testability of your Java code.

Michael Feathers wrote a FANTASTIC rule to remember when writing code to make it easy to test. His example case is Java but the rules can easily be adapted to C#. I need to train myself to remember this.

I should also get around to reading the his book that I bought.