Make sure that you take every reference material you've ever had with you. Most people I've seen going into a CFI checkride go in with 3 bags of books. This is just to ensure that if you don't know the answer, you can always look it up. This has been true for every other checkride but for this one, it is essential that you be able to find the answers you don't know. I've heard of guys getting busted just because they couldn't find the answer to a basic question. You have to understand that the FAA does this b/c they know that one day a student will ask you a question you don't know the answer to and if you can't find it (and it's relative to the curriculum), then you may look incompetent to the student.
Don't try to memorize the endorsements. I tried that and the examiner cut me off and said, "These things are always changing so why would you try to keep memorizing it?" Take a fresh copy of the AC 61-65D. Also, be able to fill in an 8710 that a private applicant would need. Be able to find other required endorsements that are not on the 8710 (3 takeoffs and landings at towered airport, etc.....).
Know the airplane really well. I was surprised that I was asked so many quesitons about the plane.
Think before you answer ANY question. Not taking the time to review your answer is the fatal mistake that many people make. If you give an answer off the cuff and it's only partly right or total B.S, then that just opens the floodgates for more probing questions about that subject.
Know the mandatory parts of the PTS. I'm very surprised at the amount of people who fail the oral b/c they didn't know the mandatory stuff. I mean, they tell you what they will DEFINITELY ask you and there is no reason not to know it.
Like someone above said, know what makes an airplane fly and what can cause adverse characteristics. BUT don't try to be an engineer. Know about stability and performance and what affects each.
Prepare for this checkride with proper technical preparation but don't forget the simple stuff like proper rest beforehand. The CFI checkride will be one of the longest, grueling of your life (if you're normal). However, it is the most rewarding thing I've ever done. Chances are you are at a large flight school and you've had good instruction. Use the tools you've learned. Put the effort into it and you'll see the rewards. Good luck.