On Selecting Literature - this is an addition to an earlier post - see below for the start everything.
Orchestral excerpts: for undergrad auditions not typically asked for, unless you are auditioning at one of the major conservatory programs. If you are not totally familar with orchestral rep, don't bother.
Prepare: Of course pay close attention to dynamics, musical ideas, shapes etc...but also know something about each composer you are performing. For example - do they have other works for percussion? Have you played them? Why did they write this piece? Was it a commission, from a book etc..? The more you know, and can share, the better.
Also, with all forgiveness to my composer/percussionist friends, unless you are a graduate student with a highly developed interest in writing music, you rally don't want to bring your composition as your ONLY solo - perhaps if you bring it with you, but make sure you cover all the bases first.
Plan ahead: The spring before your senior year you should start looking at various pieces with your teacher, with the goal of having the program set by mid summer. This gives you some time to make changes and adjustments going into the busy fall season.
A note on graduate auditions:
This is an entirely different subject. MM programs are so personal and specific it is hard to give general advice. All i can offer is this- know exactly what you want, and you'll find the right program. Chamber music, orchestral, solo, big city, small town, etc.... Get to know the students in the program, research the teachers, and look at what type of scholarship and grant opportunities exist.