There's a really useful exercise I've used to help people which I think of as expanding and contracting.
When I'm working with a student, the student will first go through the presentation. Then, informally, I'll have the student summarize what he just told me. Then, what are the three main points? The one main point? We'll chat about why he thinks those are the points are so important. This is the original full blown presentationcontracting into a few summary points.
After contracting, I'll have him expand and go through the presentation again, unscripted, but to really focus on hitting his main point. There's typically a lot of extra stuff in presentations that kinda feel like it'd be nice to say, but actually muddy the message.
At this point, I usually start asking lots of questions. What if I'm an audience member, and I see things this other way? That point you made earlier, what led you to believe that's true? What if I agree that your proposal is good, but I'm thinking of all these practical considerations - cost, time, personnel - what would you say to that? This helps the student expand at some crucial points and think things through. Now, it's always impossible to address every single possible objection, so of course after thinking through all of this extra stuff we'd need to contract once more.
One of the really nice things about the process of expanding and contracting is that it helps the student develop a really strong working knowledge of the subject. In the presentation, he can talk through various aspects of the subject conversationally because he's already talked it through.
Now, does this always work? I was once doing this with a bunch of interns, and I could see a lot of them getting more polished and talking about their topics more competently, but there were definitely still a few that just had a lot of nerves. I came across a little strong as a judge who would pick apart what they were saying after they were done, and for most of them their nerves weren't getting any better.
So, I decided to have a little bit of fun, and so we rehearsed again unpolished and without any of the nuance I kept coaching them to adopt. They just said whatever they wanted to say, I yelled at them from the back of the room (AMEN! PREACH IT BROTHER! THAT IS A FANTASTIC POINT!), and we just had a really good time. After making things fun, one of our interns who was the most nervous became one of the most confident.
When the interns gave their final presentations, they were amazing, and that was a great moment for me as their presentation coach.