All excellent advice.

Another approach to determining what your tests should do is to see what you can remove from the code.

Just delete a line or subroutine.

Do all your tests still pass? If so one of the following is true.

• You need to write another test to highlight the defect in the code you've just created.
• You've found a feature that's surplus to requirements - well done you now have a simpler code base.

God I love unit tests :-)

You can even automate this sort of thing if you are that way inclined (see this interview where Java's Jester is discussed for an example).