Though you are acting, be yourself. Introduce yourself, shake hands and avoid coming off stiff or fake.
Just like a job interview, do some research and prove to the director you not only know who he is, but are familiar with his work and style.
Study the sides and conduct a quick character evaluation prior to entering.
Ask if anyone in the room minds being addressed during the audition. Some people prefer to not be "acted to" while others do not mind.
If you flub a line or mess up, do not sweat it. Gather yourself and start over, but always ask if they mind you starting from the top.
Once finished, hand the directors your headshot with your resume on the back.
Shake hands on the way out and inquire about the followup process if not previously informed. Follow up yourself a bit later with a "thank you" card or e-mail.