For me, I never did much in the way of prep. Trying to memorize a presentation was a recipe for disaster for me. If you're truly an expert in the subject matter you're presenting, you should be able to just stand up there and start talking about it with maybe a postcard-size outline in case you go off on a tangent. It is far more important to know your subject matter then to focus on the presentation itself. If you're not 100% confident in your knowledge, that's an entirely different matter and I'd recommend a good bourbon or 3 about an hour before showtime.
The largest group I spoke to was around 400 for about 50 minutes. That was strange because it was impersonal and almost like speaking to an empty room compared to much smaller venues with 20-40 and/or courtrooms. Expert testimony was, obviously, much tougher than presenting at a seminar or conference because there was an attorney on the opposing side who aggressively contested every single word of your presentation. You won't have that so you can just BS your way through it if needed
But again, if you know your subject there's not much prep needed. It's all about self-confidence.