Gather all your information. This is the most important step. Before approaching a teacher with a concern, take the time to understand what the problem is and inform yourself as well as possible.
Approach the teacher as though you and she were partners in educating your child. This will reduce the risk of alienating the teacher and putting her on the defensive. It will also reduce the chances of putting yourself in the position of not being treated as an important part of your child's educational team.
Keep up with the ways the teacher communicates. Common ways include weekly or daily home notes, email or conferences. There may be a note in the beginning of the year paperwork that tells you the easiest ways to initiate communication.
Make an action plan. Whatever the problem is, have an idea of the way you'd like to see it resolved before you go in. Know what you're willing to do, what you think your child needs to do and what you'd like from the teacher to reach resolution.
Contact the teacher as soon as you become aware of the problem. This way things won't have a chance to spin out of control while you wait. Also, when you make the appointment and go to talk to the teacher, be aware that you've likely only heard one side of the story. Have an open mind.