It goes without saying that practice is important. It's simply not enough that you practice for a few days or a week prior to the audition. You need to practice consistently for an extended period of time. The more doubt you have about your performance and the more weak spots exist, the more things will fall apart once the adrenaline starts pumping and the nerves kick in. Having the music on autopilot will allow you to worry less about messing up and give you more freedom to present the song like you would in a performance, which is what the judges are looking for.
While it's never really advisable to show up for an audition dressed in costume (unless it's specifically asked for) you should at least dress up like you would for a job interview. As well, know the part you're auditioning for and if possible dress somewhat accordingly. There's certainly no need for a full tuxedo or an evening gown, but if you're auditioning for a slightly upper-class role, for instance, it wouldn't hurt to maybe wear a shirt with a collar and some slacks, or a nice dress. Dressing well also helps convey a sense of professionalism to the judges. For all the same reasons that you dress well on the job or at an interview, dress well for auditions.
Having properly prepared sheet music for a piano accompanist is vital. Never perform a capella. No matter how good your voice is, using the piano to help provide heft to your voice and help emphasize your dynamics can't hurt at all. If you're doing only a small section of the song, have the beginning and ending clearly marked on your sheet music. Obviously, it's even better if you have a pianist come with you whom you've worked with before, but this isn't always a viable option. Having clearly marked sheet music available is the next best thing.