For your audience to hear you, you must breathe correctly. People tend to start sentences on a high note and fade out; their voices start to sound monotonous because they don't have enough breath to support variety throughout the entire sentence. A good voice has sufficient breath so that the audience hears all the important sounds.
We get into bad habits and don't use our breathing fully. To really see how to breathe, watch babies or animals. To breathe fully, like they do, take a deep breath and observe yourself. Did you use only your shoulders and chest or did your diaphragm (the area just above your waist) expand too? As you inhale, your diaphragm should expand, and it should deflate like a balloon as you exhale.