Having experience jumping with real students (ya know, like as a coach?!?) will get your mindset right, it makes everything easier (pre-jump lesson, gear up, equipment checks, overall safety and knowing how students react to things.
Hardest for me was the ground preps, I always find it difficult to 'teach' someone that knows way more than you do about the subject (IE).
Get your Cat C and Cat D lesson plans done, practice, practice, practice. During the course they will show you what they're looking for and how they approach the training, take notes and see what you can incorporate into yours.
Use the available training aids, take the aerial picture out to the landing area and have them show (dry erase markers on laminated pictures work great) you their planned pattern on the picture, then have them show you using the actual reference points. Throw some problems in there (if you're not here by x feet, what are you going to do) to check learning and ensure that you've covered everything, handle harness (or whatever you guys have to train students on EP's with). Perfect demonstration, execution on the ground and again at mock-up. Fix everything before you get in the plane, it's too late then.
If you get the chance for the pre-course, DO IT! Nothing beats going out with your actual evaluators and practicing (even dialing in your fall rate is a huge help). That can save you course jumps and they will tell you if you're ready to go 'Hot' or things to work on.
We had a guy in the course show up with no helmet or AAD. We thought he was crazy, after the pre-course jump with spin stops, he dropped his rig, walked straight over to the gear store and bought a full face helmet.
Relax! One of the guys in the course was getting stressed out on the plane, he had 10k jumps (mostly tandems), but he was stressing ME out a little... I had to tell him to relax a few times... If you're stressing out the 'stu', you're wrong. The jumps are no time to be freaking out and rushing, slow is smooth!
I personally thought the jumps were the easiest and most fun part, was like a game... What's he gonna do now?!?! I still can't figure out how those guys got thousands and thousands of jumps, and still can't seem to pull for themselves!
I went to one of Bram's courses (then went back for my tandem rating a few weeks ago). Like he said: They aren't there to make you into AFF-I's, they're there to see if you're ALREADY an AFF-I (mentally).
Oh yeah, invest in caffeine for the ground portion...
The most difficult thing for me by a freaking long shot was believing in myself. Once the confidence switch flipped on, I had no problem with the rest of it. Practice a LOT. I probably had 35-40 practice jumps before the course. I didn't do an official precourse, but did a lot of jumps with two evaluators at my home DZ, and they were amazing with the pointers and confidence building.
Having just finished the course with Bram, I have a couple of comments/opinions. I was initially not prepared for the uncomfortably close precision flying, with the "student" sliding all over the sky. But eveyone's skills sharpened toward the course end. The course work and information overload presented staggered me at first. My brain was mush every night. At mid-point all three of us mentioned we didn't know the day of the week, or how many jumps we had made. It was always, what is the next task. Be prepared, mentally. The course director will push to the limit in every catagory. Easily the most difficult training ever. But well worth it.