***Treat every audition just as you would a performance.
For Musical Theatre, if you're not a trained dancer, try to take regular dance classes, whatever the genre of class it may be -- it will help you to pick up on the dance routine quicker.
Be sure to tell the pianist exactly how you want your piece played. I have seen this so many times, people auditioning who merely hand their music to the accompanist, then take their place in front of the auditioners and the pianist plays the song naturally how he/she thinks is best. Unfortunately, it is rarely how the person auditioning was planning to sing it.
*Sigh* Be one with the pianist. Don't compete with him/her.
Don't just stand and look pretty. I've heard some of the most fabulous singers at auditions, but they are not on callback because they just simply didn't show any acting ability. Act out the song.
P.S. Donald Oliver's "How To Audition For The Musical Theatre" is a good one, too.
"More than in any other performing arts the lack of respect for acting seems to spring from the fact that every layman considers himself a valid critic."
--Uta Hagen