Finding out you are unavailable after accepting the audition.
Check your schedule to ensure you are available for shoot dates before you accept an audition. Otherwise, you not only take up another actor's slot, but you've wasted the casting director's time as well. Never audition just for the fun of it.
Thinking the casting director is the enemy.
You may believe the casting director is trying to keep you from getting the job, but that is really not the case. The casting director is on your side. He or she actually wants to book you for the job. When the casting director is able to show his client he has found someone the client will want to book, it makes everybody look good.
Not dressing the part.
According to Backstage, you should always expect last-minute auditions. Keep a bag in the trunk of your car that contains several wardrobe choices that are well-suited for the types of roles you are most commonly called in for. For example, if you tend to find yourself on lots of auditions for roles where you play an office worker, you would want to have corporate attire on hand that you could quickly change into before your audition if needed.
Shaking the casting director's hand.
While it might not cost you the job, the casting director will appreciate it if you refrain from shaking his hand. Casting directors can see a 100 people or more a day. Shaking that many hands in rapid succession is asking for the flu. Plus, some actors are so nervous that even their hands are sweaty. Consider it a lasting impression that you don't want tto leave.