There are many approaches but perhaps few that focus on the way we breathe. Begin with the Voice Tips page.
I address the voice considerably in private session. I feel that if you are serous about voice development then you need the aid of a teacher to give you instantaneous feedback as to particular sounds and feelings.
Deeper, Natural, More Resonant Voice
Voice quality inspires others to action and improves singing and public speaking. The voice is nothing more than wind passing membranes. Breathing is the indispensable foundation. Restricted breathing causes restricted voice.
One must be able to sense, measure or assess quickly and easily to see if your breathing is improving or deteriorating. Then you must learn how to stimulate the breath without inhibiting rib cage expansion or diaphragm rise. And during that you must learn how to develop the diaphragm to stay in proper sequence and balance. Optimal breathing techniques develop big, strong, easy breathing. Your strengthened sense of self just naturally follow.