Know what they want. Many drama conservatories ask for the same types of things, so if you are auditioning for a lot of schools, you may be able to prepare three or four monologues that will work for several schools. Find out what each individual program wants SPECIFICALLY, though. Some schools want a two minute monologue, while others want something even shorter.
Prepare as few pieces as possible. Your stock monologues that you have on hand should include the following: a one-two minute cutting from a classical monologue, a one-two minute cutting from a contemporary humorous monologue and a one-two minute cutting from a contemporary dramatic monologue. Pick pieces that work as both sixty second cuttings and two minute cuttings, that way you don't have to learn an entirely new piece for different schools. The fewer pieces you have to focus on, the better they will be.
Pick appropriate material. Shakespeare wrote, literally, hundreds of wonderful pieces from dozens of plays for young men and women. Work with your drama or english teacher at school to find something unique. Also, many contemporary monologues can sometimes have extremely vulgar language and crass situations. There's absolutely nothing wrong with that, per se, but if every piece you do is veering off into the area of crass and vulgar, the audition panel may get the wrong idea about you.
Don't do the same stuff everyone else is doing. You need to be out in the auditioning community and really paying attention to what your contemporaries are auditioning with. Stay away from the monologues that everyone seems to be doing. Read ENTIRE plays and pick things that speak to you. There are so many plays other than Romeo and Juliet to take monologues from. Please, please, please do not do the Queen Mab speech. Your audition panel will thank you for it.
Practice, practice, practice. The only way to get yourself to a place of calm when you walk into that room is to know your material like the back of your hand. You must be so comfortable with the pieces you are doing that you can (and sometimes do) recite them in your sleep. That way, when you walk into that room and you start in on your first piece, it flows out of your mouth like the most natural thing you've ever spoken in your life.