There's a lot of good advice here (I love that look-to-the-right trick, I'll have to try it next time I have stage fright). When I start getting choked up speaking or singing, I visualize icebergs as a brake on my runaway emotions. Why icebergs? They're cold and unemotional, and they block out images of crying children, ravaged forests, etc. I would imagine other things would work too - anything that makes you feel cold and detached. I don't do it for the whole song, just when I need to get a grip.