Yes, this is a pretty old subject, but...
Im just the asst. Dm at my school. But when the time comes, we notify our band director about our interest in becoming a DM. (or Field Commander, what we usually call it) He provides a tape cassette featuring a part of a show for us to rehearse a couple of weeks before tryouts. You also have to create your own salute within that time. When tryouts come, usually after school, the "contestants" draw a number from a hat. If you get "1," you audition first. And for some reason, Ive always drawn a "1." But anywhoo, you have the whole room to yourself and the judges, while the other DM wannabes wait outside. They give you some patterns to conduct, cues, cut-offs, and you perform your world-famous salute. Then you conduct your prepared peice (the tape cassette) Afterwards, they as you a bunch of questions, ie, why should you be DM. The also give out a bunch of scenerios to you, and ask you to explain what you would do during those situations if an adult were not around. Stuff like that.
Once everyone has gone through the process, which all together is around 2 hours, we wait to be called into the room for our BD and/or judges to tell us the results. And then you go home. Woo.