You need to be clear about what they should do next and what they will get in return. Also, give them multiple ways to take that action. For example, if the call to action is to order your product, give them the ability to do it online, over the phone or even via a mail-in coupon. Different people have different preferences when it comes to the modality of communication. Give them multiple means of response so they can choose the one they are most comfortable with.
