Practice, practice, practice. I had the same issues early in my career, then turned it around so that I feel comfortable on the phone, web, in person, talking to 1 person or speaking in front of 500. I was able to overcome those issues by first, knowing the material completely, secondly writing it down, and lastly saying it out loud, wash, rinse, repeat.
First, I believe that knowledge leads to confidence and with a deep understanding of the material the words can flow from you, instead of you having to think about what to say next. So know what you are presenting inside and out.
Secondly, Write it down. For me, this process allows me to "work my words." By writing it down, I get it out of the vagueness in my head into clear ideas. This can be an outline or simply just a stream of thoughts, there is no right way, it's about just putting the thoughts to paper.
Once it's out of my head and on paper, I say it out loud, to make sure everything sounds right. If it doesn't I go back to the writing and continue to work the wording until they are my voice and flow naturally.
Lastly, don't hesitate to pull others into your practice session, use your friends, spouse or peers, as a sounding board. They don't have to know the subject, because they'll be able to tell if you are confident or not.
Presentations of any sort when "nailed" are exhilarating, feel free to set up a call to dive deeper.
