I'm 100% talking out of my ass here because I've never been in a bordercross race. However, I did race back in the mid 90's a little.
Rule #1: Have fun. If you take it too seriously you'll get psyched out. Think of it as a fun way to spend a day and the results are just something to look at at the end of the day.
Rule #2: Get in shape. Yes adrenaline will help you for the first few runs, but when you're going full tilt for several runs a day it'll catch up with you and you could get hurt. Do squats, jumps, etc. to make sure your legs aren't your limiting factor.
Rule #3: Be consistent. Get into a routine of pre-race nutrition (doesn't need to be crazy but a healthy breakfast that's easy to digest will go a long way in any sport), gear setup (try to always have your boots the same tightness, your edges the same sharpness, etc. You don't want to have to learn to ride dull edges in the middle of a race.
Rule #4: Look at rule #1 again...
Good luck! Any courses to practice on there? I've got a local course to practice on, gets bloody icy though cause everybody and their brother goes through there.