Get the boat ready, bring the spare parts and spend as much time on the water as you can. That being said, my feeling about a Nationals is that the mental game is the part that is so hard to prepare for.
You probably haven't raced in a fleet that size and certainly not with the competition you will face.
Know the rules on the start line and at the marks. This will come into play constantly. Try to get a clean start and sail clean. It's a big numbers game. If you feel as though a good finish for you is a 25 then don't tack out of 20th place. Throwing in some 35 or 40's will mess up your position in a hurry.
I have been trying to get this right for a long time and haven't made it through an event yet without posting at least one big number. It ain't easy.
My goal the last two years sailing with my daughter has been to post a number in all the races. We have managed to do that in some tough conditions.
I finished 75th in my first Nationals. Your sure to beat that.
Good luck and have fun.