BE PREPARED!!! That's the best advice anyone can give you.
You will most likely be nervous...and I know that it is hard-but TRY to relax, the casting director WANTS you to be the one they cast. They are on your side!
Make sure that your sheet music is clean & marked off where you are beginning and ending in the song and that any "adjustments" you have made in the music are clear and legible. (point them all out to the accompanist prior to singing as well) Perhaps if you haven't already, put all music in individual plastic inserts in a 3-ring binder. It keeps it clean and (usually) the accompanist prefers it in the binder as it is (usually) easier to turn the pages while playing. Bring more than one song that you are comfortable with(!!!) If you do not have an audition book put together yet...compile your sheet music of songs into that binder that you can COMFORTABLY sing and can sing at a moment's notice, whether you were prepared to sing them or not, as they might ask for another song.
Get a good night's rest the night before...any rehearsing you do at 2AM the night before the audition isn't going to make much of a difference on how you will do the next day. So it is better to be well rested. Drink lots of water, as well.
Wear something bright, but not distracting that compliments your skin and hair. A color that YOU feel great in. A memorable color can help them remember you, in a good way...unless it is an obnoxious color like neon yellow or something.
Be comfortable in your style of clothing. You don't want to be fussing with and pulling up or down anything while waiting for the audition and/or DURING the audition. A nice top & skirt would be fine if that is what you are comfortable wearing. (I am assuming they told you that this is just a singing audition and that you won't be dancing until callbacks further down the road...) Also, comfortable shoes!
Limited makeup-what you would wear on a daily basis, really. Accent your best features. Hair should be clean and out of your face. A hairstyle that you won't be fussing with the whole time.
Also, I personally always HINT at the character with a certain style skirt, hairstyle...something along those lines. Do not go in dressed AS Tracy. Go in looking like YOU, but with a hint of Tracy.
Smile and be polite to ANYONE you come in contact with inside (or even outside) of the building. You never know who knows who and/or what they will tell them. Introduce yourself to everyone when you walk into the audition room and be sure to thank your accompanist and whoever else is in the room for their time.
Best of luck with your audition....believe in yourself!!! Let us know how it goes. :)
