hey
after having done lots of orchestral auditions, my tips are;
1. look confident, if the panel think you're ready to show them your best, then you might just play to your best.
2. find the right piece of music.
make sure it shows your best playing, but try to pick something that you've worked on recently, exam piece etc. If you've played it recently, you should still remember where to come in with all the piano entrances, and it should be of your highest standard.
3. know your piece. if possible, get a recording or two if its a famous concerto movement etc. if not, ask your teacher for stylistic advice if they havent already told you.
4. work on sight reading. i know its the last thing you want to do, but if you're a good sight reader you'll be a great player for a good orchestra. find any music you havent already played before, look on the web for audition material for sight reading (im found a whole web site dedicated to horn excerpts so there must be one for flute).
tip; if you're bad at tonguing, pick a fast-ish piece so that they'll give you a slow piece of sight reading. sounds like crazy logic, but it worked for me and i got into a national orchestra!
5. if something goes badly (hopefully it wont!) then dont let the panel know. no screwed up face or sighing. this is especially important for the sight reading, imagine a top pro sitting in a performance, and giving a great big sigh and a

to the audience!
6. smile and try to look happy. enjoy it (if you can!) cos if you wanna make it big you'll have to audition lots for orchestras.
if you get it, thats great, but dont rub it in the faces of your peers who didnt. equally, if you dont get in, its not the end of the world, and im sure there'll be an orchestra in your area that will be happy to take you!
hope this helps
hornplayer