As a voice teacher and acting coach, one of the largest hurdles I see my students face is that they fail to realize that it is the SAME VOICE that shouts and yells as that which sings or does a monologue. Why can they make these powerful, amazing sounds doing a visualization with me in the studio yet once they start to sing or speak, it disappears into a small, cautious tone? The mental adjustment needed is a simple one - to connect the dots between the two voices and focus on your Chi.
Here is a simple exercise but one that I've found to be very effective: visualize that you are grounded to the earth, immovable like a giant oak tree. Feel your centre of power in your solar plexus glowing bright yellow like the sun and imagine that your voice is flowing like a bright ray of sunshine out of your mouth and into every corner and crevice of the space you are performing in. As you sing or speak, see your voice hitting every single corner of the room, under the furniture and behind the curtains. Your voice will feel big, expansive and powerful as it flows from your body and fills the room.
With a little practice, you will be able to tap into and duplicate this technique in public at will. Try it! The results might surprise you.