Simple Tip for Good Tone

Bring the harmonica to you, not you to the harmonica.

Translation...

Players often bring the harmonica close to their lips and purse their lips to meet the harmonica. This pursing raises the jaw and makes the entire mouth cavity (your resonating chamber) small... this results in poor tone and tired facial muscles.

By opening your mouth and bringing the harmonica into your mouth, then all that's needed is a slight closure of the mouth to create the seal (like a soft gasket) needed to play the harmonica... this results in better tone and less fatigue.