I talked earlier about the typical hi-tech presentation where the content is largely on the slides. In that case you must add color by what you say rather than simply reading what is on the slides.
The alternative approach is essentially to make a speech. The real content is in what you say. The slides then should be graphical backup (pictures, graphs, key points) to what you are saying. Watch a Steve Jobs keynote from MacWorld (example) to see this type of presentation done really well, or presentations from TED (but beware, not all of them have slides at all).
But just like Steve Jobs or the TED presenters, to carry this off well you need to rehearse until you have your speech perfect, either basically memorizing it or doing it from notes. Whatever you do, don’t write it out word for word and read it. The slides are not going to help you remember what to say, they are another complication for you to make sure is synchronized with your speech. So rehearse it without the slides until you have that perfect. Then rehearse it with the slides. Then rehearse it some more. Like a good actor, it takes a lot of repetition to make ad libs look so spontaneous.
This approach will not work presenting to foreigners who don’t speak fluent English. There is simply not enough context in the visuals alone, and your brain has a hard time processing both visuals and speech in a second language. If you know a foreign language somewhat, but are not bilingual, then watch the news in that language. It is really hard work, and you already know the basic story since they cover the same news items as the regular network news.
If you are giving a keynote speech, then this is the ideal style to use. You don’t, typically, have a strong "demand" like you do when presenting to investors (fund my company) or customers (buy my product). Instead you might want to intrigue the audience, hiding the main point until late in the presentation. So instead of opening with a one-slide version of the whole presentation, you should try and find an interesting hook to get people’s interest up. Preferably not that Moore’s Law is going to make our lives harder since I think we’ve all heard that one.
I find the most difficult thing to achieve when giving speeches to large rooms of people is to be relaxed, and be myself. If I’m relaxed then I’m a pretty good speaker. If I’m not relaxed, not so much. Also, my natural speed of speaking is too fast for a public speech, but again if I force myself to slow down it is hard to be myself. This is especially bad if presenting to foreigners since I have to slow down even more.
I also hate speaking from behind a fixed podium. Sometimes you don’t get to choose, but when I do I’ll always take a wireless lavalier (lapel) mike over anything else, although the best ones are not actually lapel mikes but go over your ear so that the mike comes down the side of your head. That leaves my hands free, which makes my speaking better. Must be some Italian blood somewhere.
Another completely different approach, difficult to carry off, is what has become known as the Lawrence Lessig presentation style, after the Stanford law professor who originated it. An example is here where he talks about copyright and gets through 235 slides in 30 minutes, or watch a great presentation on identity with Dick Hardt using the same approach here. Each slide is on the screen for sometimes just fractions of a second, maybe containing just a single word. I’ve never dared to attempt a presentation like this. The level of preparation and practice seems daunting. I’d be interested if anyone else has any experience of trying this.