Christian
(New Zealand)
As a solo pianist and guitarist, I have always had a great desire to be able to sing louder than the instruments actual sound, without beginning to strain, or sound "whispery" or use a microphone.
After some time of practice, I began to see that by focusing on the voice alone, and working on simple pronunciation exercises, that I was able to increase the volume a great deal. I do still struggle with certain aspects such as vocal fatigue, straining, etc, but am finding that practicing without stress tends to eliminate most problems.
My tip is – believe in your ability to sing! Anyone who can speak can sing, and believe me – if you dedicate only fifteen minutes a day to focused practice, you will definitely begin to see results.
Good luck my fellow musicians!