I was always impressed with one of my past IT managers at how he could "wing it" when explaining the latest services his teams offered in front of a hundred people. He also happened to be a pastor so I figured he had learned something I didn't. I asked him if he had any secrets of the trade, and I'll never forget his answer. He said "Yeah, I know what I'm talking about." So simple, but don't forget to really know what it is you're explaining.