Warm up your voice. This is probably the best advice you can follow. Warming up before you audition can help ensure your voice is smooth and your vocal cords are relaxed. Scales are an ideal way to warm up. Sing a combination of vowels and consonants as you practice your scales. Also be sure to sing to the full extent of your range, from the lowest note you can achieve to the highest, singing each scale ascending and descending.
Familiarize yourself with your part of the song you will audition with. Though sight reading is an important skill for anyone singing in a choir, it's a good idea to have your piece memorized. This can help prevent mistakes in sight reading. This doesn't mean you should not be prepared to sight read. A choir audition might include a piece you've never seen before, specifically for testing your ability to sight read music.
Find someone who is auditioning with you and work together. Find either someone who sings in the same range as you do or even in a different range. Practicing with another singer is an effective way to prepare for the harmony singing that is often a part of choir auditions. There's no guarantee whether you'll be singing with others or alone during an audition, but it helps to be prepared for both circumstances.