Respond quickly and use appropriate grammar when responding to emails.
No email should be left unanswered for more than 24 hours.
Appropriate grammar is important so make sure any auto-responding emails you have being sent out are proof read along with any regular emails.
For auditions always arrive on time.
Being too early can annoy some casting directors and being late is unacceptable. Arrive to the neighborhood early and find a small coffee shop to look over the sides or copy at until its time to arrive at your audition. Remember to plan for late trains, personal hold ups, etc.
This emphasizes your professionalism.
You will outshine every other actor who arrives late and frazzled.