My approach to voice training is to build the muscles involved in singing, strengthening the instrument, and lengthening the vocal cords for maximum range, projection and endurance. I focus on developing the technical aspect of the voice first and foremost before approaching repertoire.
The result of this approach is a voice that can sustain singing for several hours without getting fatigued, a voice that can fill an entire concert hall with ease, and a voice that has a range of three octaves and more (usually four and more).
Building the voice in this fashion allows singers to approach a wide variety of musical styles, from pop, musical theatre, jazz, and classical including opera. Furthermore, this approach can even repair voices that have been destroyed by belting, pushing, and other unhealthy methods.
This is not a quick and easy fix, but rather a systematic development of the entire vocal apparatus. It takes time and effort, but the results are simply fantastic.