Probably the biggest breakthrough while Unit Testing is to realize that writing tests offer priceless feedback about the design of your production code. This feedback goes from suggesting immediate obvious improvements, up to hints about some of the most advanced design principles in software engineering. This article briefly enumerates these hints, pointing to questions to ask about your design.
Disclaimer: Don't follow blindly these hints, but make sure you understand all of them so that you can decide in your real problem what would improve the design, and what wouldn't.