Memorize the words. One of people's biggest pet peeves are listening to a singer butcher the words of the National Anthem. Get a copy of the lyrics and commit them to memory.
Practice daily. The more you practice singing the National Anthem, the better your final performance will be. Set aside some time each day leading up to the event.
Work with your accompanists. If you have a keyboardist or band playing with you, set up regular rehearsals. You want to get in sync before you sing the National Anthem.
Keep it simple. There is no reason to add your own arrangement unless asked specifically to do so.
Learn all the notes. Although the National Anthem has a simplistic melody, you should attempt to hit all of the notes accurately. Find the key that you are most comfortable in if need be.
Wear something respectful. A National Anthem performance is not the time to bust out the bustier and leather pants. Conservative dress is much more appropriate.