This is a great thread -- so many useful insights.

Here's a new tip from a Ph.D in choral music: breathe in, as if you were sucking air in through a straw. This sends air down deep, right "to your diaphragm." Now reverse this flow to sing. Ta da!

A lot of people who assume they always sing in tune do not, actually. They get off, just like everybody else. So don't get overconfident, you "stars."

Singing can sound bad, not because one is out of tune, but because something (such as running out of breath) caused the tone of the voice to change. In other words, one accidentally went from chest to head tone, or from nasal to resonant. People don't like to listen to this. I think not running out of breath helps you avoid this problem. Also, listening carefully to yourself.

Cupping your hand around an ear helps you hear what you are actually singing, and it doesn't look as silly as sticking your finger in. I do this in church (inconspicuously, I hope) when the instruments prevent me from hearing myself.

Another piece of good advice I got - join a regular choral group.