There is a very famous TED talk that suggests that stress is only bad for you if you believe it to be so. Now that is a very empowering idea.  

And when my kids are nervous about a swim meet or an upcoming spelling test; or when I'm getting ready to speak in front of an audience, I think about this talk and I tell my kids and myself that the butterflies in our stomach, and our heart beating faster -- this is our amazing body getting ready to perform.  And that we have to trust that it will perform brilliantly because we've prepared for that moment.