According to Speaker Rich Hopkins, you should look out for the "forks in the road" of your speech. If your audience is strongly reacting to your message, build on that message. If you get a huge laugh from your audience, consider wrapping up your point then (even if you have more jokes planned). Be flexible in your speech and aware of your audience's reactions.
Reacting,
Tim Wilson
Professional Speech Coach
Free speaking tips at: http://speakingquicktips.blogspot.com
