DO NOT use a mirror. If you use a mirror you will be thinking about what you look like, and it will take you out of the moment. Write a full biography of your character. Go through your monologue and underline the most important words (stress words) in each sentence. If it’s Shakespeare, and it’s in iambic pentameter, it’s already done for you. Paraprase each sentence in your own words. Write your subtext (what your character is thinking as he/she speaks.) If it’s Shakespeare, there is no subtext. Avoid choosing monologues that require an accent. Choose an age/gender/race appropriate monologue. Read the whole play so that you know the character’s story. What was your character doing 5 seconds, minutes, hours, days, months, years ago? Details, details, details. “Generality is the enemy of all art” so says Constantine Stanislavski. Stay in the moment at all times.
What are you trying to get the person you are talking to to do(this must be a strong, specific action)? What are your obstacles? What are some ways you can get this person to do that?
Arrive to your audition early. If you are early you are on time. If you are on time you are late. If you are late, don’t even bother showing up. Be dressed neatly and cleanly. Keep your hair out of your face. Be warmed up, physically as well as vocally, so you can use your body and your words clearly and effectively.
Break a leg, be prepared to do crap tons of work/writing. Also, I highly recommend This Book.