If you're anything like me, you'll be a bundle of nerves at your first race. Don't listen to Brassballs, you've gotta get in the groove for a DH race. Take your time on your trial run, it will help you be faster in the race. Look at all the possible lines. Figure out where you might screw up and walk back up and reride that part of the course until you're comfortable with it.
When you do start, realx and feel the flow and you'll be much faster than if you hammer it right out of the gate. Go as fast as you are comfortable with-this is your first race, there's always time to win next time after you have some experience. Racing DH is about being smooth, consistent and fast, in that order. You have to know how to brake, how to turn and how to jump with speed. Practice, practice, practice.