As a parent, you are the expert on your child's behavior. You might not know why he does the things he does or how to remedy challenging behaviors, but you know what his behavior is like day in and day out. Others won't know your child as well, so it's sometimes helpful to write a letter to caregivers or medical professionals describing the behavior. At the very least, this letter can serve to open the lines of communication to help you work with the others in your child's life.
Outline the major points you want to highlight. You probably won't have enough space to detail all of your child's idiosyncrasies, but you want to hit on the behaviors that most deserve mentioning. You might mention that she needs to stay active while learning, and might need to squeeze a ball during lectures, but leave out twiddling her hair when she's telling a lie.
Address the letter formally. If you're writing to his teacher, you might say, "Dear Mrs. Gilman," but if you're writing to his psychiatrist, you might say "Dear Dr. Wood."
Start the letter with a brief introduction describing why you're writing. You might say something along the lines of, "Since you'll be working with her throughout the year, I wanted to let you know about some of her behavior issues and how to avoid them."
Incorporate details to describe the behavior issues. You might say that your child tends to withdraw when someone uses a raised voice with him, or that he tends to hit only the children who are smaller than him when he's frustrated. These details will give the person a more complete picture of your child. He may not be acting quiet because he's shy, but because another child just yelled at him.
Explain the behaviors, if you can. In some cases, you'll know why your child is acting this way, or what sets her off. You might mention that you've recently moved and that she's having potty accidents at home due to the stress, or that her self-appointed classroom helper role comes from a desire for adult approval.
Offer any solutions you might have. If you have techniques that work at home, let the other person know what you do so that there is some consistency. If you know that a time-out always leads to bigger tantrums, you might mention this and that you have better success with redirecting to a different activity.
End the letter with an invitation for further discussion. You might say, "Please feel free to call me to discuss any concerns you have" or "Let's schedule a conference if we don't see improvement within a month." Include your phone number or email address and sign the letter with your full name.
This letter will likely become an official part of your child's record, whether at the school or the medical office. Typing is better than writing by hand, since it will be easy to read. It should look professional and neat.