Breathe evenly. Take it slowly, and don't rush.

deep breaths :) hope it goes well

thanks you guys, I just got done with it and although I was a bit shaky at the beginning and completely blanked on some information (i had to look back at the slides to remember) I survived and am feeling better now

Chew gum beforehand. According to studies, the action of chewing actually calms tense nerves (which is a reaction to stressful situations) because it tells our brains that if we're "eating" then we shouldn't be in danger; thus, feelings of adrenaline (pounding heart and heavy breathing) should subside.

Whistle "twisted nerve"

Plan a joke during the presentation, it will lighten the mood.