Rehearse and rehearse and rehearse until no longer do you have panic (you are anxious, you can't breathe properly and you can't remember what happened) but you do have perspective (you have become the observer of your own performance). How long does it take? The tough answer is more practise than most people are willing to do; which is a shame as you'll never fear a presentation again.
Fast-track your progress with the book.
Awesome Presenting 1: It starts with why.
Awesome Presenting 2: Don't blame PowerPoint
Awesome Presenting 3: Be Clear on the Goal
Awesome presenting 4: Think Movie
Awesome presenting 5: More Stories
Awesome Presenting 6: Brilliant at the Basics
Awesome Presenting 7: Beginning-Middle-Action
Awesome Presenting 8: More Spikes
Awesome Presenting 9: Rehearse
Awesome Presenting 10: Add Value
Awesome Presenting 11: Don't 'mash'
Awesome Presenting 12: Get There Early
Awesome Presenting 13: Be Different
Awesome Presenting 14: Review, Learn and Improve.
Awesome Presenting 15: Not a script, a storyboard.