I do speak publicly from time to time and certainly get butterflies like everyone else. Two things that always help are overpreparation and - this is REALLY important - engaging the audience. Make it interactive.
In a couple of weeks I'll be conducting another fitness seminar. I have 1:15 allotted. I plan to inform the audience that I am just the warmup act and THEY are the principal speaker. I further plan to inform them that I get to speak the first 15 and they HAVE to speak for an hour.