Don't allow your posture or your tone of voice to signal impatience. Allow the writer to take his time in exploring the issues that he wants to explore. Engage the writer by being a good listener. Make eye contact, and wait for the writer to finish a thought before jumping in with your questions or suggestions. Respect the writer.
Nothing inspires trust like respect.
Keep track of the writer's responses to the session. If the student seems to be confused or distracted or bored, try a different approach. There's no room for sarcasm, superiority, aggression, or criticism in a tutoring session. The way you close a session is as important as the way you open it. Too often, tutors rush the end of the session, feeling the need to get to the next writer on the sign-up sheet.