What I do an awful lot is use some microphone technique. Hold the mic close to your chin just below your mouth and listen to the feedback. Move it closer on quieter passages and move it away when you get louder.

Also remember that a very small moment of silence can sometimes work wonders to make a noisy audience quieten down.

If all that fails then stand up and take a really deep breath and sing something that's well within your vocal range.

And if you're singing and find that you are just not going to make that note, then SHOUT it instead of singing it.