Voice-over training requires a lot of talking! If you are not used to projecting your voice for a continuous period of time, your vocal chords may tire easily. [Teachers: you have a leg up on all of us!] Your vocal chords will eventually adapt to the added demand you make on them as you practice controlling your pitch, attitude, volume and energy. If you find that you feel a bit tired or winded after practicing voice-over technique, maybe you are not using your diaphragm. Learning to use your diaphragm will not only increase your stamina, but it will allow you to control your pitch, attitude, volume and energy more adeptly, and put you in the perfect place to get more voice-over work!
Your diaphragm is a smooth muscle that lies beneath your lungs and controls the mechanics of inspiration. You are always using your diaphragm to inhale and exhale because it is a muscle that works automatically, like your heart. When you inhale, your diaphragm actually contracts to allow air to be sucked into your lungs. Your diaphragm then relaxes, which puts more pressure on your lungs so that you exhale. While your diaphragm does this without your conscious awareness, you can deepen your breath, calm your nerves, and control your voice by consciously engaging your diaphragm.
First, you need to find your diaphragm! Do this exercise to learn where your diaphragm is and what it feels like when you consciously control it. Start by lying on your back on the ground. Place a book on your abdomen and breath easily for a few minutes, just like you normally do. The book should rise and fall with each inhalation and exhalation. Next, watch the book rise on an inhale, and this time when you exhale try to keep the book (and your abdomen) raised like it was at the end of your inhalation. You'll feel like you are pushing down into your stomach when you exhale in order to keep your stomach raised, which is why this exercise is sometimes called "deep belly breathing". Continue to keep the book elevated with your abdomen as you slowly inhale and exhale.
You should feel a definite increase in your lung capacity after doing this exercise for only a few minutes. Try starting each voice-over job or practice with this exercise, and see how you feel after only a couple weeks!