Just fresh out of the 2009 season so I hope some of this helps:
I can't speak on the behalf of other corps, but one can safely assume that the core traditions and values are similar in choosing drum majors. Firstly, there's no need to over-think the "audition" process. Sure, some corps have a cute little system built up where auditionees conduct, then run a metronome, then stand in front of the potential corps members... Some corps restrict the position to veterans only.
At some point in my "dm-ing", Brian Howard told me that conducting is 5% (maybe) of the entirety of the position.
And without doubt, he was right. Basically, don't waste too much time focusing on the "ribbony," "glamour" aspects of the position. Conducting and keeping time (unless you suffer in this category miserably) should be second nature... you should be able to bust it out with little refinement. So, get your pattern down, be comfortable, make sure you can keep consistent time, and lastly, check if you're panting after 15 minutes of straight 150 bpm 4-pattern (1 lbs wrist weights help a LOT). If you know you're going to be doing some side or back-field conducting, get ready... you're going to need to learn how to ignore what you hear and watch the floating hands.
Next, get out of your shell. It is a bold yet truthful statement that some people don't ever "blossom" in time to step up to the position. Thus, some people aren't meant for the position.
I tell this to all music leadership students that I work with, but basically, as drum major, you are the right and left hands of your staff and corps director. You need to smell the burning forest (as so to speak) before anyone does (corps members and staff alike). Just prepare for a whole lot of mental nullification. In terms of physical duties, yes, yes... you are the first one up, last one to go. But it transcends beyond that at a certain point. Just be prepared... you are the corps "ambassador" - stuck in the limbo of corps membership, yet authority - the bridge between members and staff.
Your sole purpose is to be a catalyst to your rehearsals, to ensure the satisfaction of your staff, members, and director - to ensure that everything that can be done in your disposition be fulfilled to make things run smoothly. You are the wild card, the one who can pick people up at train stations, the one who paints the fields, the one who understands the psychology of each of your staff members (what he/she likes). You are also like a salesman a businessman who tracks down (literally on paper [i find this helps a lot]) every task that is given to you. You run the metronome, you regurgitate instructions.
I re-iterate, get out of whatever shell you have. Be confident :D
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Lastly, a little bit about the dreaded Dr. Beat:
- Find one.
- Master it.
- Make staff smile.
- Corps members don't frown because they don't reset.
Even in metronome usage, you can distinguish a drum major who is on top of it because he/she will always be anticipating. Remember, you want to make rehearsals as fluid as possible for your staff. As soon as they (the staff) are done with fixing things here and there, boom, it's time to go.
Tip: If during an extended segment of the show (especially with music) you "screw up" on the met, be prepared to shut it off, communicate with your other on the podium, and come back in on time. No drum major can go about saying he/she hasn't screwed up once on the met the entire season... It's just how fast you can recover and fix things that matter most.
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Ending note: have you ever read The Giver back in middle school? If not, wiki it. You are the giver of your corps: one who takes info and makes a sacrifice for the good of everyone else.
The sad thing is, I can list all these things and yet 80% of it you will learn to understand through experience. As far as getting selected, go in confidentlly, emit a clever intelligence, and know your duties.
Make people smile; delegate; be a politician. Be a leader.
Gosh darnit, and I wrote up there "don't over-think."
Good luck.
-John (BD)