Elements of good poster design

Audience - Who are you trying to reach with your poster? What are they interested in knowing about this subject? How would they go about finding the information?
Purpose - What is the purpose of your poster? What type of information are you trying to communicate?
Message - What is the lesson you are trying to convey?

Design - What type of design is going to appeal to the audience, make them focus on your message and still be true to the purpose of your poster?