There are a couple of ways to do it, but to be honest, the simplest solution would be to change the way you are output it. If you are asking this question, then you are probably using one of the point based overloads. Instead of your current DrawString, look to use one of the overloads that takes a rectangle:
http://msdn.microsoft.com/en-us/library/21kdfbzs.aspx[
^] for example.
The system will than handle the wrapping for you.