Attend concerts. Because a cappella concerts are often small and intimate, you'll be noticed in the crowd if you're a common face. Show that you're a fan.
Learn the music. If a member of the group were to ask you what your favorite song is from their repertoire (and someone probably will), have a good answer. Be prepared in case you're asked to sing part of it.
Practice singing. You won't be expected to know the group's latest CD by heart. Simply practice singing the music you like. Join a campus choir to keep your voice limber. Know your vocal range.
Introduce yourself. Make yourself known to someone in the group, especially if you share a class or activity with her. You'll be easier to remember. If the group hosts any social events, attend and introduce yourself to as many members as you can.
Prepare for the audition. You will most likely be asked to sing a song of your choice. Choose a song that plays to your strengths, fits within your comfortable vocal range and allows you to show off your personality and enthusiasm. Practice reading music, singing a scale and repeating notes you hear--these are common audition tests.
Show up for your audition. That's the easy part. You may be surprised, though, how many people get cut for coming late to their audition.
Show off your personality. The way you interact with members of the group is just as important as your singing talent. Most importantly, be yourself.
Make yourself available. You may have to audition for several rounds before you can join. Show that this group is a priority for you and clear your schedule during audition times, if possible. However, don't skip class for an audition--the group can most likely work around your schedule.
Don't panic. If you forget the words to a song, take a deep breath and try again. They'll want to see that you can bounce back after making a mistake.
Be open minded. You may be asked to do something wacky... like make animal noises or create choreography to go with a song. These types of suggestions test how you react to pressure and how willing you are to open yourself up to your audience. Don't take yourself too seriously and you'll pass with flying colors.
Don't get discouraged. If you don't make it into the group, try again for the next round of auditions. Sometimes it's more about timing and what vocal ranges are needed than about your talent or personality.