When running 1 car in three classes (IIC, Snowbirds) I found it useful to create a checklist for each class, and refine it as the week went along. I include things like "check wheel nuts" for spec classes where I'm swapping tires every race, usually near the bottom.
Whenever possible, I leave pinions on the motors they're to be used with.
Arrive at the race with your gear ready to hit the track. If you dig a hole, it's hard to recover. Avoid unnecessary maintenance for the same reason.
Sorta related: Never underestimate the importance of sleep when trying to perform at a high level. Read a book on it if you're doubtful.