If you're testing already-written code, I'd do it top-down, testing overall results first, then working to more granular levels. If you're writing new code, I'd do it bottom-up, module-by-module, function-by-function.

Here are some posts that I've bookmarked that you might find useful:

• Wanted, more simple tutorials on testing
• Test driven development and glue code
• Functional and Unit Test Redundancy
• Application Testing: Custom Module or Perl Test Files?