Use your own personal stationery to compose your letter. When writing to your teacher, ensure that the document in no way looks like an affront. A personal letter will be read more agreeably than a letter that looks to be all business.
Greet your teacher and state your issue or request after a suitable lead-in. For instance, if you are asking for a letter of recommendation, state that you are beginning to apply for places in various universities and would like them to assist you in that endeavor. On the other hand, if you are writing to your child's teacher to let her know that your child was unable to complete his homework, state that events the previous evening made it impossible for him to finish the assignment.
State your reason for writing. If it's because your instructor knows your strengths better than your others in the case of a reference request, let her know this. If it's because you're hoping that she'll understand the stressful situation in which you found yourself, make your concern known. Additionally, if you want to let her know that your child doesn't understand the material being taught, suggest that a review of the material might be in order.
Thank the teacher for her time, and state that you look forward to speaking to her soon. Sign the document by hand, even if you typed the body of the letter.