Tim... you need to do just one thing... breath!!
I just took my CFI ride back at the begining of the year, so I remember all too well the feeling of anxiety and fear of the unknown that creeps up on you the day that it gets set in stone what day the ride is going to be. Unfortunately that feeling normally don't go away until the day of the ride, but there are a few things you need to tell yourself.
1) You know your stuff. Your instructor isn't going to sign you off and send you out to the checkride if your not prepared, and teaching to the level you need to be.
2) Look thru the PTS and make sure you have lesson plans on every single thing listed in there. If its not in there, it's not fair game for him to ask. Sure he can ask if he wants, but he would have a hard time justify failing you for something that he can't reference on the pink slip. Remember, the PTS is a "Grading" rule book that both you and him have to follow.
3) Like I said earlier, just breath and relax a little bit. Something each Examiner told me on each of my 3 CFI rides: "I dont care as much that you aren't flying the manuever as proficiently as you probably can, it's already been proven in previous checkrides you can do the manuevers; what I want to see is that you can TALK me thru the manuever, and critique me about how I'm doing." Another examiner always preached "if its silent, and your not talking in the cockpit, then your not earning your money as a CFI. Even if it's straight and level flight, always be asking him questions or scenarios." The more you talk, the more excited the Examiner's get on the Instructor rides. And TALK about evert detail, no matter how minor (taxi centerline, taxi wind correction, runup information, who we call when on frequencies, etc.)
4) Just have the attitude on the day of the ride that no matter what the outcome is going to be, your just going to give it your best, and if he finds a deficient area and gives you a pink slip - then you can go home, study up as much as possible ont hat area, and blow him away next time. The way I always looked at it, is once you fail a checkride on the oral phase, when you go back you normally dont have to do 3/4 of the oral that you had preciously studied for. Just go in and clean up the deficient areas. Plus there usually more lenient on the second try, because they really dont want to fail you a second time if they aren't forced to.
GOOD LUCK, and I look forward to hearing the battle report after the ride on Wednesday!