Be sure to ask writers immediately what sort of help they need.
The more you know about their writing issues, the better you can be of assistance. You can then focus the tutoring session on the specific challenge the writer has been facing.

Try different tutoring methods until you find a technique that works best for you.
Some tutors read drafts silently, but others prefer to read them aloud to the writer. Some tutors prefer to make comments as they go through a piece of writing. Other tutors like to read a paper in its entirety before saying anything to the writer. Don't be afraid to try different techniques! You will be the best tutor you can be when you feel comfortable with your technique.

Let writers know that you can't "predict" or "promise" them a good grade.
Some writers think that visiting a tutor will guarantee them a higher grade. You need to let them k now you can help them learn to recognize and improve upon theirwriting challenges, but you cannot promise them a good grade. They must earn their grades on their own!

Don't let a student complain to you about a teacher and/or writing assignment.
This creates a negative environment and is an extremely inappropriate discussion to have. If a writer starts complaining, stop him or her immediately and explain that you would just like to focus on the assignment and any writing challenges.

If you ever feel uncomfortable in a tutoring session, address the issue right away.
For example, if the writer directs any sort of rude or disrespectful behavior toward you, ask that he or she stop. If the situation does not improve, then you have the right to end the tutoring session immediately and refuse to tutor that writer in any future sessions.

Resources

Purdue University Online Writing Lab (OWL) is a great resource for students and teachers.

University of Texas at Austin Writing Center contains an exhaustive listing of online sites for documenting sources in the fields of humanities, anthropology, archaeology, history, social sciences, journalism, sociology, biology, biochemistry, molecular biology, chemistry, chemical engineering, civil engineering, electrical, computer engineering, mechanical engineering, and petroleum engineering.

Wisconsin-Madison Writing Center contains useful information pertinent to the writing process. It also has links to documenting sites and writing centers.

Rensselaer Polytechnic Institute Writing Center contains excellent handouts on technical subjects such as electrical, computer, and systems engineering.

Dave's ESL Cafe has many useful resource links to websites.

Associations

Midwest Writing Centers Association has links to writing centers in Illinois, Iowa, Kansas, North Dakota, Michigan, Minnesota, Missouri, Nebraska, South Dakota, and Wisconsin. Very useful!

International Writing Centers Association provides useful information for starting a writing center, directing it, and setting-up online help.