Sounds silly but it's only when we ask "why?" that we can focus on what, how and to whom.
The more narrow your definition, the better you will be able to target your message, therefore the more effective your communication will be.
Are you focusing on the benefits to the audience? Too often we write about things we think are important (Ex.: Few prospective students care how old your college is; they do care about your low student/faculty ratio.)
Have you chosen the best medium for your communication? We tend to respond to a communication need with a print publication. But it may not be the best medium. Consider "push" email, advertising media, a telephone campaign or the Web.
Have you made it easy for them to take that action — by including a postage paid reply card, toll-free telephone number or e-mail address, for example.
You will need to consider how you will handle phone calls, mail response, filling requests for information, taking reservations, etc.
If so, will it be the first of a series, or must it stand alone?
Either way, make sure your audience knows who you are and how to reach you (Web address, email, postal address, telephone number).
Ex: Billboard or poster copy must be very short. The purpose of both is first to catch your attention and second to deliver a strong single message. If you have 200 words on your poster, it is ineffective. If you have more than 5-7 words on a billboard, there isn't time for the auto passenger to read it. (Remember "Got Milk?)
We know that the more personal the communication, the more successful it is. Ideally, the best communication is 1:1 - so if your audience is small, consider telephone. But even if it's not, do what you can to include their names, their stated preferences, their location, or other personal information in your communication.