You have to know who you're talking to, what you're talking about and keep the presentations and simple and as conversational (very important) as possible (if your information isn't at least mostly formal, there's no need for it to be dry and monotonous).
What I like to do is rehearse what I'll be going over in my head before I do the presentation. This DOES NOT mean I go over line for line what I want to say. I go over each point I want to make and just go off of what I know.
For example, if I have to talk furthering the development of Africa's rail infrastructure, I would obviously have to cover:
- Financing
- Potential issues with corruption
- What wouild be done about infrastructure that's already in place
- etc
Each point needs to be thoroughly went over in your head before you say it so that nothing important can be missed out on. Besides, if you remember what your points are and know what information encapsulates said point, you can easily make as many variations during your speech as you want to. You'd be surprised.
I also can't stress this enough: keep your presentations as conversational as possible. Your audience needs to be engaged throughout the presentation, even if you're not actually having a conversation. They'll be more receptive to your ideas if they feel as though they're being included in the discussion. What works really well too is being inquisitive and trying to get their opinion on any specific matter to make it seem like you know what you're talking about. Saying "how many of you have ever......" works very well to keep the flow conversational.
Just take this simple advice to heart and I'm sure you'll do well.