SeanM pretty much has the way of it. the tines have got to rest against the teeth so as to use skull conduction for your sounding board. the tines are held in position by lip pressure. The mouth becomes the air column.
A little tip I learned from Rolf Harris is that if you open the airway and let the air column come from the diaphragm it's like adding an amplifier!
The third thing is not to pull the striker too hard just flip it lightly with the back of the index finger.