my voice coach taught me to sing scales and single notes on a "zzzzzz" or a "vvvvvvv" to build muscle tone. it takes a lot more strength of diaphragm to push the sound through, and if you then sing the same notes on an open vowel, your volume will have doubled!
if you do this, however, be careful not to _over_ do it. it's like any other kind of athletics; jumping in too quickly can lead to injury (hoarseness). you'll need to build over time.