You'll tell stories at your best if you prepare not only your story but yourself. Your voice and body are your instrument, and it helps to use them well. To project and sustain your voice, you must breathe deeply and correctly. To check this, place your hand on your stomach. As you inhale and your lungs expand, you should feel your stomach push out.
Many people do the opposite, holding in their stomachs and breathing only with their upper chests.
Also be sure to keep your back straight, so your lungs can expand fully. To avoid strain, relax your throat and jaw muscles, and the rest of your body as well. A big, loud sigh will help this. Pronounce each sound of each word distinctly. Tongue twisters are good for making the tongue more nimble.