Many people focus on "the high note" because it gets the majority of the attention and applause however, the key to getting the "high note" night after night, and really "growing" the voice, is really a biproduct of strengthening the "mid-voice" and connecting the "Passagio" (or passageway). (This is that funky froggy sound which sounds like a yodle when you are going from your "normal" chest voice into your head/falsetto voice).
When this area called the passagio is connected correctly, and strengthened, you will find that you now have one long powerful note and the "high note" is no longer "feared."
If you keep your throat completely open (starting on the ah vowel sound) and work simple 3/4 to one octave scales, maintaining the same velocity (don;t sing one part louder than the other) and carefully connect through the passagio you will begin to grow this area. You MUST start gently, as you will probably have a difficult time connecting the two at first. Gradually, you can add more "weight" into the sound to grow this area.
Support and losing the tension is critical as well. Constantly monitor yourself.
The vowel sounds will need to change as you go up the register.
This will be another subject for another post.
For actual physical singing examples of this, please feel free to check out my posted videos here on Modern Vocalists or visit me at www.KenTamplinVocalAcademy.Com for more info.
Great singing success to all.