A tip I was given a while ago that I find helpful in projecting your voice without expelling all your breath: Instead of trying to sing out to the farthest point in the room, imagine the sound of your voice is at that farthest point and that when you sing you are drawing the sound towards you. It sounds strange, but give it a go.

Another tip is to choose a short ditty and try to sing it as many times through as you can in one breath. After a few gos you naturally start to learn how to sing without blasting all the air out in one go.

Good luck

Bagpuss