Sure, sure! I don't know if I'm too late or not, but if you make it this might help anyway:
1) If they give you a routine or fundamental to practice before the tryout, see if you can borrow a flag from the school and practice daily. The more mastery you have, the more confident you'll feel.
2)If you mess up during the tryout, recover quickly and don't let it get to you. Everybody makes mistakes and it doesn't mean you've ruined the whole routine if you drop or forget.
3)Make yourself look as confident as possible during the tryout, even if you have no idea what you're doing. Stand up straight and keep your chin up.
4)NEVER look at the judges. They will look very serious and menacing, even if you are doing a wonderful job. Look above their heads at the wall and focus on what your hands are doing.
5)If you drop your flag at the LAST toss, spin, or lock-out in the entire try-out (not, I repeat, NOT, in the middle of the tryout...see number 2 for that situation) do not pick it up, grimace, or move in anyway. Look as though nothing happened until they tell you to leave the tryout room.
As a last note, freshmen tryouts into colorguard are actually the least stressful of all tryouts during colorgaurd. They give you a very wide margin for mistakes, to take a deep breath and do the best you can.