I would say a great pre-cursor (if you don't know how, so you may already know this cold) would be to figure out how to plan dives and really, honestly use dive tables. I know that this isn't nitrox specific, but being nitrox qualified does mean people will expect more out of you as a dive buddy. You should know how to calculate surface air consumption, respiratory minute volume, and how to figure out what size cylinder you need for a particular dive. NWgratefuldiver (Bob Bailey) has a great web site that covers all of that material here:
Bob Bailey's site
Bob also has a great article on nitrox mythology on there as well.
Being a nitrox diver implies that you should know how to choose when or if a mixed gas like EAN is appropriate for the dive length, maximum operating depth, or required surface intervals. I've seen plenty of people pay 15 bucks for an EAN fill when they didn't need it and could have been diving compressed air for the 20 minutes that they wanted to dive at 30 feet. I think if you know how to properly plan, prepare, and execute regular compressed air dives upside-down in your sleep with just a pencil and paper, you'll smoke the nitrox course. Furthermore, you'll be a real nitrox diver when you're done and not just some guy with a card. You'll know when to use it, and why you're using it.
Brushing up on all the gas laws wouldn't hurt either. If you know Boyle's, Charles's, Dalton's, Henry's, and Martini's laws, you'll have a leg up when going through the course material. Specifically, Dalton's and Henry's laws will help you when planning nitrox dives, figuring out MOD, calculating partial pressures, figuring out best mix, etc.
I hope I didn't bore you
