There are many kinds of auditions, each with their own rules. An audition for a formal group such as a city orchestra, big band or professional touring act may require formal dress. An audition for a rock band does not (although the band will react better to someone who demonstrates a particular sense of style.) If you're auditioning for a formal group, ask what to wear to the audition. If you're auditioning for an informal group, such as a local rock band, find out what they wear and match the style as best you can.
Being in a musician is demanding, and the demands depend entirely on the group. Some acts perform nightly, some every weekend. Some groups stay local, while others travel for great distances. Before you audition for a group, make sure its demands match your level of availability and dedication.
Do your homework. Well before the audition, ask the bandleader what you will be required to play, and make sure you can play it. To do otherwise is disrespectful and a waste of time. Even if the audition is loosely structured (such as for a jam band), make sure you've prepared adequately by knowing the band's style of music.
A surprising number of musicians assume it's fine to drink or use drugs before an audition. They claim it's all part of the musician's lifestyle, or that it makes them loosen up and play better. While some bands are perfectly fine with drinking before or during rehearsal, many also are not, so it is best and safest to assume that they are not.
Chances are the band you are auditioning for is trying out many musicians, and is running rehearsals on a tight schedule. Showing up late will throw that schedule off, annoy your prospective bandmates and worsen your chances of getting the job.
Many musicians have special skills or talents: A knowledge of the music business, connections with people who can help, or extensive experience performing, touring and recording. If you have a skill beside raw musicianship, make sure the group knows it.