Taking a breath is one of the most important techniques to learn when playing a WIND instrument.  So many musicians never reach their potential because they never learned to breath correctly.  To take a huge breath, sit up in a chair as straight and tall as possible.  Totally relax your shoulders and DO NOT allow them to raise.  Take the biggest breath you can through your mouth.  If you are doing it correctly you will notice the following things:

Before you begin, I would like you to pay attention to a few things. Make sure you are sitting with your feet comfortably touching the floor and with a straight back. You should feel that your throat is open and that you can breath comfortably. This is the posture that you should always sit in when you are playing the clarinet, or any musical instrument.