Open the mouth wide while singing. Use the pharynx as an instrument and allow the jaw area and throat to retract and open, mimicking the function of a megaphone.
Relax the back of the tongue when singing. Also, do not allow it to rest high in the mouth. The front of the tongue should be just under the ridge holding the bottom front teeth.
Work the tongue separately from the jaw while singing. Doing lip exercises will help with control in this area.
Practice vocalization up through the pharynx and throughout the mouth. Use vibrato when singing by "wobbling" your pitch throughout the range. Start from down low in the throat, almost into the chest. Feel your voice rise, until you feel the edges of the tongue vibrate. This is especially true with certain sounds and in the "high" range. Continue until you feel the bridge of your nose vibrate and the voice being to ring.
Form the shape of your mouth so that your lips are flat, high and narrow. Resonance is increased when the shape of the mouth is round, again mimicking the megaphone shape.