-Work on your timing with a met. The front ensemble is not exempt from perfect time and there always seems to be one person who hasn't learned that who shows up to audition.
-Make sure you are not just practicing scales, but also ways to move through them - chromatic, circle of fifths, circle of fourths. Also, learn your modes.
-If there is a packet, know it backwards and forwards. Have your technique set as close to the description as possible, but still be prepared to make changes on the fly.
-Finally, this is important, if you don't get offered the instrument you want to march, march anyway. You'll learn more playing glockenspiel or synth than you will playing video games all summer, and you'll be much closer to that marimba or vibe spot next year.
Good luck!!