There's a very interesting TED talk related to this, by Amy Cuddy (title: Your body language shapes who you are) - http://on.ted.com/Cuddy

Basically you can manipulate yourself into being calm by taking on a relaxed and confident body language. Sitting up, stretching out, taking in a lot of space, ..

I tried this several times and it does help, of course it doesn't make your stress vanish entirely.