I am untrained but got a reasonable volume in my voice by practice and more practice at maximum volume on the way to work in the car, using deep breathing, as suggested by Morticia and Geoff the Duck. I increased my lower range more or less in a similar manner and by attempting to sing base harmonies in unaccompanied song sessions. I also believe in standing up, as advocated by PoppaGator.
What, me LOUD?
"I like to rise when the sun she rises...!"
Alan