I'm thrilled when I hear about physics teachers using student presentations and formal student-led discussions as teaching tools. Students should come out of our classes with the ability to explain physics orally as well as in writing. But that's easier said than done.
Think back to your high school (or college) days. How did you feel about listening to your classmates when they did formal presentations? (Not how did you feel about giving presentations -- since you became a teacher, you might have liked that just fine.) Did you look forward to presentation day? I doubt it. Possibly you were neutral, taking the "at least we don't have to listen to the teacher for an hour today" approach.
More likely, you dreaded those classes. Student after poorly-prepared student with minimal public speaking skill, some reading (badly) straight off of hastily-written notes. Aargh, it was academic torture.
So don't make your own students suffer. If you're going to do formal presentations, prepare for them with tremendous care and detail.
Set time limits, and enforce them. Set a timer on your phone with a loud alarm at the end. The students must know going in that if the buzzer goes off before they're done, they're done anyway. No pity, no remorse, no exceptions. Don't yield to the temptation to allow a student just to finish a thought, or to give them just a bit more time because they didn't quite get to the important part of the presentation. You're teaching a life skill here. Let them fail - then next time they probably won't fail.
Presentations can be very, very short and still be useful. Even a long-term project can often be summarized in three minutes. In fact, it is a valuable skill to learn how to communicate complicated ideas in such a short period. What makes student presentations so painful is often the attention they give to irrelevant details, while the audience rolls its collective eyes saying "arrgh, get to the point."
Do not allow powerpoint. Slides are too often used as a substitute for substantive communication. See also this post.
Practice, practice, practice. Ideally, you'll go through the class for several days before the presentations, watching each student in turn and giving feedback. You'll also have students giving their presentations to each other for days before the actual event, giving each other feedback. Make a video recording of students speaking - it's painful but important for students to see themselves droning on without eye contact, repeating themselves without communicating anything important. Force students to see these mistakes while they still have the opportunity to correct them before presentation day.
If nothing else, assign presentations for homework - require three dry runs in front of another human. I mean, you know darned well that typically students try to wing presentations without appropriate preparation. So require that preparation in and out of class. Give the students a homework sheet that asks for the signature of the person who watched each presentation. If each presentation is only three minutes without powerpoint, that's not a burden at all - it's a ten minute assignment.
Too many teachers throughout the years have assumed that their students know already about basic speaking-to-an-audience skills, have assumed their students will be self-motivated to practice and perfect their presentation. Then when the horrible presentation starts, everyone is embarrassed, just like when the kid at the talent show didn't practice his clarinet solo and now has no choice but to honk on. It's our job to insist on the practice that we know teenagers won't likely do. It's our job to teach the speaking skills that students haven't internalized yet.
Eyes up. The first thing I do in practice is start making faces at the students who stare at the whiteboard or at their notes instead of making eye contact with the audience. Pretty soon, the whole class is helping each other keep their eyes up. This good posture subsequently encourages students to speak naturally, telling their story to the audience rather than talking to themselves.
Who is the audience? It's not good enough to tell the class "you will be speaking to your classmates and two other science teachers - all of whom are familiar with the first-year physics that you have learned." It's not even good enough to explain what that means in front of the class. You must allow the students to make mistakes in their assumptions about the audience, and then learn from those mistakes.
For example, a student will write an equation like d = vt, then spend 45 s of their three minutes umming and hmmming through an explanation of what d and v and t stand for. It's never occurred to this student that the audience is familiar with this equation. That all they need to say is "We use d = vt because the cart's speed is unchanging." That the audience is more interested in the physical prediction made by this equation, and how that prediction is verified experimentally.
So tell this student right now! Right now, your feedback is in context. You're not droning on about something that your quite intelligent and experienced-at-school student thinks that she already knows. You're gently correcting a serious error immediately after it's made. Your student can't tune you out, or say to herself "whatever, I know that." She's just screwed up. If you speak firmly and with love, showing with your whole being that you are helping prevent a mistake on a bigger stage, your student will listen, and appreciate your helpful feedback.
And the message will spread throughout the class. The student you just critiqued will, in turn, share that feedback with the person she watches for practice. And everyone will get better, as a team.