Writing Great Unit Tests: Best and Worst Practises

A suite of good unit tests is immensely valuable: it documents your design, and makes it easier to refactor and expand your code while retaining a clear overview of each component’s behaviour. However, a suite of *bad* unit tests is immensely painful: it doesn’t prove anything clearly, and can severely inhibit your ability to refactor or alter your code in any way. How can you be sure to write unit tests that contribute positively to your project and don't end up being a liability?