The single best thing I learned about testing from Schwern is to make sure that every time someone finds a bug, they write a test to show the bug before they fix it. This works at the beginning of a project if, as you add feature requests to your list, you write tests for them as you write the code to implement them.

Hence, don't get bogged down in thinking about every possible thing you could test, just get started writing some tests and write more for every missing feature or bug.

Phil