One of the most valuable assets of an announcer is a pleasant voice.
Your voice is a wind instrument: It’s made from the air you breathe. A polished voice is supported by a firm foundation of air, resting on a flexible membrane below your lungs called the diaphragm. The diaphragm is like a "shelf" of muscles and tendons near the bottom of your rib cage that separates the chest cavity from the abdominal cavity.
Your lungs are shaped like triangles, with a narrow point at the top and a wide base at the bottom. Because the lungs are narrower at the top and wider at the bottom, your lungs hold most of the air you breathe near the bottom. At least that is the case if you fill your lungs by breathing from the diaphragm. But over time, people develop poor breathing habits. Rather than fill their lungs to capacity by breathing from the diaphragm, they use only a portion of the lung's capacity. They become "shallow breathers," using only a small portion of their lungs' capacity.
Breathing from the diaphragm helps you maximize your lungs' capacity. It can fill up your lungs fully and quickly. For good breathing, think about how a person breathes when they are sleeping. If you watch a person sleep, you will notice that their belly expands, rather than their chest. This is because of the action of the diaphragm.
It can also help reduce the sound of your breathing, which is an important consideration when you are using a microphone.
Diaphragmatic breathing can also help relax you. Taking a deep, deliberate breath from the diaphragm is an excellent way to reduce "mic fright" or "stage fright."
But perhaps most importantly, breathing from the diaphragm can help you achieve a dramatic improvement in the quality of your voice. It is the foundation upon which rests all of the other "building blocks to a polished voice" discussed below.
Keep your upper body straight. Think about a string pulling up the top of your head as if you were a marionette. If your upper body is not straight, it will restrict your breathing and the action of your diaphragm.
You do not want to cave in you chest as that restricts that area. Think about pushing your shoulders back. On the other hand, do not be straight and stiff as a board. You should be relaxed. Find a happy medium that works.