Totally agree with Rainbow. I'm a university lecturer so speak to groups of 20-200 people most very week. The key thing is to know what you want to say, and to be interested and excited b? it yourself.
The other thing I'd say, as a lecturer, is that structure is key if you are talking for more than about five minutes. With the best will in the world, it is hard to listen to someone else talk solidly for 15+ min. This is actually why repetition is so annoying, as you think bloody hell, you've made that point, get on with it.
So I normally start an hour-long talk by saying I will give a little background or context and then make 3 (ish) key points. I then refer back to these throughout: "so the first major point is X, and I'm now going to move onto my second major claim, Y". It keeps people oriented.
And don't, whatever you do, patronise people with silly games or questions or graphics, like having a picture of someone frowning with a question mark above their head to indicate that something is confusing. Grown ups can handle an idea like "confusion" without having it illustrated for them. That kind of twee jokiness always indicates to me that someone is unsure of their material and desperate to get the audience onside - whereas if you are straightforward and confident and concise, people will respect you.
Quite the essay, sorry :)