From phrases.org: Used by U.S. actors (though it may have originated in England or Germany) to wish each other well just before they go on stage. Actors have a superstition that saying "Good luck" straightforwardly would tempt the gods to subvert the wish by making something bad happen (paraphrased from Eric Partridge, "A Dictionary of Catch Phrases American and British").
:up: ... though I believe even an English actor might prepend 'Go' if the meaning was 'Go [out there and] break a leg' (in the situation of someone encouraging an actor with 'stage fright').
We just say "Break a leg" unless it's being used as Bob suggested. You're all set, you've prepared well, the audience is going to love you... now go break a leg!