Well, this is the area I struggle in too. But what I do to stay calm is take 5 minutes before my presentation and just meditate. I get myself in the right mindset, slow down my heartbeat, relax my breathing. Of course, I end up covered in sweat by the end of the presentation, but at least to start, I am somewhat cool and controlled.
After the presentation I do the same thing - take another 5 minutes of private time, calm down my breathing, and generally just try to move on. Normally I'd try to dwell on what I did wrong, what I could have said better, etc... But recently I've started just trying to let it all go, because at that point it's over and there's nothing I can do to change it.