Find out as much as you can about the audition process and the role for which you are auditioning. If possible, read the script in advance. Find out what will be asked of you during the casting call -- do you need to have a monologue prepared or will you be doing cold readings, or "sides," from the script? If you do need a monologue, choose one that it similar in style and genre to the role of which you are auditioning.
The evening before the audition, plan what you will wear the next day. While it is not suggested to dress "in character" for the role you are auditioning (such as wearing scrubs to audition for a doctor role), it is fine to dress to "suggest" the part. For example, if you are auditioning for the role of a librarian, you would wear something more conservative and demure than if you were auditioning for the role of a bartender. Your dress will help the casting directors see that can you look the role.
Also, pack your audition bag with the essentials you will need for the day. Include multiple copies of your picture and resume, as you never know how many directors might ask for a copy. Don't forget to pack paper and a pencil in your bag as well, as you will want to take any script notes given and write down the names of the people auditioning you in order to properly thank them later.
Go to bed early. Although it seems like "common sense," many actors take for granted that they should be well-rested for the day of the audition. Not only will it help you look your best, your mind will be fresh and ready for whatever challenges the casting directors might throw at you.
Arrive at least 20 to 30 minutes in advance of your audition time in order to calm your nerves and rehearse any sides from the script given to you for your audition. By being early, you may also learn valuable information about the audition from the other actors while you wait. Be extremely nice to everyone you encounter. Remember, it may be the receptionist who greets you who actually has the ear of the casting directors. While you wait, rehearse your prepared monologue or memorize the script given to you. If you cannot completely memorize the sides, at least make sure you know the first and last lines. It is important to not be looking down at a script when you are trying to grab their attention in the beginning of your audition or at the end, when you want them to remember your face and not the top of your head. After auditioning, be sure to thank the casting directors for the opportunity.
After your audition, immediately send a thank you note to the casting directors who auditioned you as well as anyone else who was especially helpful to you during the process. A hand-written thank you note will remind the decision-makers who you are and show your professionalism. While you may not be the right person for this job, they will remember you for future roles.