Ebbie, what helps me out is to repeat a little saying:

"Anything worth doing is worth doing badly."

Whenever I get nervous about getting up in front of a group I start thinking of all the ways I could really screw things up: mangle the lyrics, spit on someone in the front row, have my pants fall down, fall off the stage, etc. etc. Pretty soon I'm laughing at the thought of TRYING to do all the things I'm afraid of doing by accident, and the jitters go away.
Works for me.

Elmer