I think everything I could say has been said - I play flute and it is all down to breathing. It takes some practice to get it right though - if the breathing isn't quite right the low notes will sound 'fluffy' and the high notes will be squeaky! I suppose the way they describe it in books as 'blowing harder' is sort of right - in the simplist form anyway. But it's also to do with how you position your lips and how you control how fast or slow you are breathing out. I sort of do it automatically by now but I think it's if you blow faster it tends to be louder.

xLx