Give yourself a physical anchor onstage, meaning pick someTHING on the set, or on your costume, and everytime you feel the butterflies rising, physically touch that thing. It will simply ground you, and give you a sense of familiarity and comfort. It can be anything, like a button on your sleeve, or a coffee mug on a table, but actually put your hand on it, take a breath, and keep going. If you get nervous again, repeat. It's completely mind-over-matter, but it's always worked for me. Oh, I realize now, that you may not even be an actor but maybe are asking from the point of view of a musician, or public speaker.but I'd imagine the idea would still be the same. I'm an actor, so that's where my mind automatically went.