Speak with the director. In community theater, directors will usually notify the people they wish to see again a day or two before the callback audition. When you receive the call, be professional. Even if you're excited beyond belief, save the screaming and jumping around for your friends and family---after you get off the phone. Be sure to thank the director for considering you.
Make sure you understand all of the details about the callback before hanging up the phone. Know where the callbacks are being held, on what date and at what time. The director should tell you which role you have been called back for, and what you will be expected to sing. You should also find out if any dancing will be required at the callback audition.
Research the show if you're not familiar with it. Become especially familiar with the role you've been called back for. You can find detailed synopses of most plays and musicals on the Internet. If you have access to iTunes or another music service, consider downloading a song or two so that you're familiar with the musical style of the show.
Pack for the audition. Bring extra copies of your headshot and resume (the director should already have both, but it never hurts to be prepared). Bring dance shoes and appropriate dance attire if a dance audition is required.
Warm up in the car on your way to the audition. Arrive early and sign in. Turn off your cell phone, or at least set it to vibrate.
Think about your character before singing or reading. If you've researched the show and the character, this shouldn't be difficult. If not, familiarize yourself with the song and reading material as quickly as possible to get a feel for the role. Be in character as much as possible when singing, reading and dancing.
Thank the directors and the accompanist before leaving.