Likewise, to the OP. My advice will be to do much exercise, particularly exercise which lowers the heart beat and everything else, thus when you get nervous in such occasions, you won't be half as nervous, your heart won't be pumping to such crazy, unnerving extents, and you won't be shaking that much or to an extent which is tolerable....
Otherwise, you could go to the doctor if this is a serious problem, it sounds like it is....they could prescribe you propranolol - which helps calm down the onset of nerves, useful for people who get panic attacks in such situations, me myself being one of them, perhaps your best bet is decent exercise blended with propranolol as an 'insurance policy' as such...i feel it's far more honourable a thing if you at least make some effort to exercise, as effectively it reduces your nerves in the body...whilst using propranolol in small doses...
Stating the obvious, it obviously helps having prepared well for a presentation, got into a right mental, physical frame of mind et cetera....but some people can have big phobias about something like this, so it's certainly not plain sailing even when you have met basic/common criteria for making a success of things.