In addition to the King CFI knowledge test course, I'd also like to recommend the "CFI Practical Test Course", which shows John King taking a practice CFI checkride with a real examiner. It's quite thorough and gives you a good feel for how the practical test will be conducted.
As a CFI applicant, you could be asked to teach anything in the Private or Commercial Practical Test Standards. The CFI checkride DVD (or video) doesn't actually cover EVERYTHING you could be POSSIBLY be asked, but watching John King talk his way throught the oral and flight test should give you some good ideas on how to approach the ride.
Good luck,
FFFI