Look for errors in your test and product logs periodically.
This doesn't have to be difficult; usually a simple find and grep will work. But go around the error Usually you'll find small things that can be easily cleaned up - failed attempts to copy files that you deleted from your repository, warnings about permissions when moving files that leave a few logs behind, etc. Occasionally you'll find really large things, although hopefully this is rare. A simple find and some cleanup, though, will help make sure your tests continue to do what you expect them to do, and continue to test what you expect them to test. Think of it as spring cleaning, tester-style.