Just want to add a few more things from my experience:
Try and work on your car during the day at the race track, finish as much as you can on the car, motor, setup...etc. while you're still at the track. Try not to work on your car at night and get good rest. I find that I drive much better when I'm well rested.
Work out a schedule that will allow you to maximize your time while working on your car. For example, glue your tires first, and while that is waiting to dry, you can work on cutting some motors, or cutting out a body.
Lastly, if possible, have a race buddy who is not racing the event help you out (charging batteries, gluing tires...etc.) and also look over your car while it is on the track (check your driving lines, and the handling of the car) It is a tremendous help when you have someone helping you out. My good friend MDawson on this board helped me out recently at the TCS National, and that was the best finished I ever had (4th overall in GT-1) and I credit a lot of that was due to his help in the pits
Good luck to you at the big race!