So how do you know what messages what topics what sub topics are going to be of the greatest interest to your audience? I have a radical suggestion for you. Just pick up the phone and ask them if you're giving a presentation to 50 people at a trade conference on Friday, call one or two of them and just ask them what they think people would be interested in, ask them what they're interested in, and you could perhaps list 20 ideas, see which three or four jump out at this day of information being so accessible and everyone googling everything and going to Wikipedia people often forget the simplest way to get information sometimes, it's just pick up that phone and have a brief three, four minute conversation with someone. Now if you are leading some kind of a seminar and you have a database of people who are supposed to be there.
You can also send out a questionnaire in advance. list your 20 possible messages or topics of themes. Ask people to list three or four they're most interested in or put them in priority. also have an open ended question. Ask them what you would like or what they would like you to address. Your best source of information quite often is your actual audience.
You can't wait till you're in front of them and ask them but you can communicate in advance. It's not hard to do e mail posted on a blog for comments or just pick up the phone