In my experience the best tests have the structure of a specification. Some flavor of test-first discipline is the easiest way I know of to achieve this. This w

In my experience the best tests have the structure of a specification. Some flavor of test-first discipline is the easiest way I know of to achieve this. This way when requirements change, you change the tests such that they match the new specification. Sometimes this reveals that large chunks of the code need to be thrown out and replaced, but that's OK.