Be prepared. This is the single most important thing you can do. You must know your music. At an audition, you want to play the music to the best of your ability. If you are having problems with the music at home, just think of what might happen when you are in an uncomfortable situation. Work with your teacher on the materials. Play a 'mock audition' for your family or friends -- sit them down and play through your audition materials. Get used to playing with people staring at you.
Plan ahead. Make sure you have left yourself an adequate amount of time to prepare the music. Read the application and audition requirements completely and carefully. Even if you think you know all about these things, re-read all materials.
Know the 'big picture'. If you are preparing orchestral excerpts, make sure you listen to a recording of the pieces. When playing orchestral excerpts it is important to know how your part fits in with everyone else's.
Get a good night sleep. Try to do everything the day before as you normally do. Do not go to sleep much earlier than you are used to, as you will end up tossing and turning and getting a worse night's sleep.
Eat properly. Do not eat foods that are spicy and upset your stomach. Some people say that eating a few bananas the day before an audition will help your nerves. If you are too nervous to eat, make sure you....
Hydrate. Drink plenty of water the night before and the day of the audition. Some people get dry-mouth when they are nervous and this will help.
Think positively. When you think about the audition, visualize yourself going into the room with confidence and hear yourself (in your mind) playing the best you can. Do not think of the audition as some scary horrible thing. The audition is something that you should be excited about. You want to 'say' to the panel "Look what I can do!" not "uh, I hope I don't mess up". Focus on your playing and your confidence.
Wear comfortable clothes. Make sure you know if the audition is behind a screen or not. If it is behind a screen, you can wear whatever you want that makes you feel the most comfortable. If the adjudicators will see you, then wear a suit, shirt and tie, dress, or nice skirt and blouse. Look neat. Do not wear perfume. Make sure you are comfortable playing in the clothes you choose to wear to the audition. Can you sit and play properly in that skirt? Does the jacket restrict your bow arm? Can you stand in those heels?
Leave plenty of time. Make sure you have directions to the audition location. Look at them then look at a map. Be sure of where you are going. Think about where you have to go, when you have to be there, and which roads you must travel on. Then decide what time to leave. Make sure your car has enough gas. Check to make sure you have your instrument and all necessary music before you leave. Make sure you have a contact number to call if you get stuck in an unexpected traffic jam.
Warm-up. If possible, warm-up before you leave home. Then plan to arrive at the audition location about ½ hour before the audition. Sign-in. Find the rooms you need to know: the audition room, the warm-up room and the bathroom. Don't listen to others practicing their material and don't listen at the door of the audition room either! You cannot control what others do, so forget about them. Concentrate on playing the best YOU can.
In the audition room. Keep breathing -- you can't audition if you faint. Play what the adjudicators ask. If it is a screened audition and you have a question, you may ask the monitor a question to ask the panel. Remember that in a screened audition, you are not supposed to talk to the panel. Your audition may only be 5 to 10 minutes long. You will not be asked to play every single thing you prepared. The adjudicators will select what they want to hear. In some situations, they will ask you to pick the first excerpt or solo. Think about this ahead of time in case you are asked this. The adjudicators are not your enemy. They are nice people who want you to play great! As for sight-reading, yes you can practice for it! When you practice, take an etude book and flip it open to a page you have never seen. Give yourself a minute to look the music over, and then play it! It helps if you can do this with someone, who can check to see if you made any mistakes. Sight-read one etude a day, it will help you get used to playing new music.
No one will ever accuse you of being too musical or playing a piece that is far too interesting. Sometimes in an audition, the adjudicators ask you to play something again with some sort of change (play louder/softer, play more staccato/legato). This doesn't mean you are playing badly; they may want to see how you take correction. Even if you think you are already doing what they are asking for, do not argue, just play the section they are asking for and really emphasize what they have asked you to change.
After the audition. Don't freak out. People like to beat themselves up after an audition. Be realistic with yourself about how you played. Look at the overall impression you left. Think about what you did well, and think about what you did not do well. Write some notes down that may help you for the next audition you take, whether it is about the music you played or your preparation. The audition is not just an audition it is a learning experience.