You've (hopefully) done all the necessary training. Preparing for the big race day, though, involves more than just logging miles in your running shoes.
I've been running (and racing) for over 14 years. Needless to say, I have learned a lot through trial and error (emphasis on the errors).
A little planning ahead of time can ease the race day stress. It may not eliminate the start line nerves, but having a lot of the details ironed out before the big day will make getting there a little easier.