Tip of the Day: Beginner Tips #4
Now that you have your hands, lips and tongue set into a relaxed position...
Place the harmonica a little deeper in your mouth, lower the middle and back of your tongue (the tip-top is on the harmonica, but the middle and back are able to move interdependently to lower for good tone), drop your jaw a bit, and finally open your throat.
A simple way to perform all of this is to use a slight yawn... it will place everything in the same position.
What's stated above are the elements needed for good tone. Joe Filisko just stated in his latest series of lessons that the harmonica easily produces a thin, weak and tiny sound, but with the elements above we can overcome this.