Make sure you understand the subject matter inside and out. Every math book is slightly different and each edition will tend to introduce its own way of looking at problems. If you are teaching algebra, for instance, you should pick up several textbooks by different publishers on the subject. Make sure you are familiar with how each one approaches the specific concepts and that you can easily solve the problems typically included at the end of each chapter.
Come prepared to a tutor session. You should arrive with everything you could possibly need including paper, pens, pencils, a calculator, a protractor, a compass, graph paper and anything else the specific subject might require. If it is your first lesson with a student, you should have an introductory form with questions to gauge the precise difficulties, the goals and the current grade situation.
Take time to understand the student. If you are just beginning with a new student, you should sit down with him and thoroughly discuss the difficulties he is having. Some students and parents seek a tutor simply out of lack of motivation in doing the work because the student does not like math. Other students develop problems with a specific new subject in math that they have never had. A number of students have just never done well at math and need help with every new math concept.
Sit down with the student's parents to discuss what the expectations are with the tutoring process. Figure out the frequency of sessions, any history of learning disorders, any motivational issues and the end goals for each part of the school year.
Ask the person being tutored to figure things out as much as possible. One major error tutors make is that they simply take over the problem once the student gets stuck. It is important to push the students abilities as far as they can go. Don't try to jump in right away. Tell them to take their time and give them the confidence that they can do it on their own. If a student has repeated difficulty with a specific kind of problem, have her solve easier problems again and move up slowly in more complex problems. Let her understand that if she can solve the problems at home, she will have no problem whe taking a test.