It'll be easy to only think about music and practice your butt off--which is certainly necessary. But it's necessary to be visually/physically prepared as well.
Up until this past year, good players at the Bluecoats have been able to get away with poor marching. When I auditioned, I was elated to hear Fanning say that marching would be a larger part of the audition process than ever before.
In addition to basics, we learned some sample drill, and had to play an excerpt from Tilt along with it. During both basics and sample drill, the staff is testing how fast you can learn things as well as how well you follow instructions and adapt. One of the best moments of my visual audition was when a staff member asked what my dot was from two sets before and I spat it out with no hesitation. Additionally, they may quickly go to fast tempos and big steps. No matter how good or bad you think you look during these times, always act like you're the best marcher in the room. They notice kids who take challenges and perform well.
And as said many times before, don't worry about numbers. Plenty of kids will show up but you'll have no idea how many are auditioning just for experience, and you won't be able to assess the skill level of others--it's needless worrying.