Figure out what type of learning works best with your student. Different people learn better using different methods. Some people need physical examples or practical experiences to learn, others learn better by hearing while others need to see and read the information. Try different methods until you find the one that works best with your student.
Find the difference between work that is difficult for the student because he struggles with the work, and work that he is not doing well on because he's disinterested or not really trying. This helps you decide whether or not you need to simplify the work, or to find ways to make previously boring work engaging for the student. Get excited about the material and you can create passion for it in a student, and help him believe in his own ability to succeed.
Talk to your students. Understanding what their personal goals are and what they see as their difficulties helps provide a yardstick for you to evaluate their progress and current abilities.
Create tests or get old tests fin the subject that you're tutoring someone in. When tutoring someone for a actual class, she will eventually have to demonstrate her knowledge in a test-taking environment. Give her practice tests to evaluate how much the tutoring has worked, what areas need to be focused on further and whether some time needs to be spent focusing on her test taking skills as well as on the subject material.