Make sure you find out exactly what you are supposed to accomplish when you are asked to do a public speaking engagement. It is best to have it in writing. This can be handled with a question on your pre-program questionnaire. If you don't get this information, you are open to complaints that you did not achieve the goals of the meeting organizers.
P.S.: Just because this is a short article doesn't mean you should ignore it's BIG message. Ignore this point and it can mean BIG trouble.