How to overcome my fear of public speaking?

Ever since I was in my early teens I've always had a fear of speaking in front of a large group of strangers. I'm 26 and every time I have to do a presentation for cl or work I get very flustered. I feel like I can't breathe, my mind goes blank, I get tongued tied and my heart beats a mile a minute. I want to beat my fear because this semester I have 6 presentations due through out the summer.