Fellow speaker,
A great way to get people to respond to the Q and A part of your speech is to allow the audience to talk amongst themselves before going into the Q and A. If you are doing a keynote, limit this discussion to 1 minute maximum; for a day long seminar this discussion can be considerably longer; and if you are doing a 2-day seminar the discussion could go on for as long as 45 minutes to an hour.
Discussing,
Tim
