Here are two threads regarding Unifieds tips. One is from 2012, but still relevant information.
http://talk.collegeconfidential.com/theater-drama-majors/1374475-please-coach-me-about-unifieds-p1.html
http://talk.collegeconfidential.com/musical-theater-major/1924770-chicago-unifieds-audition-scheduling-advice-and-best-practices-p1.html