This blog post by Copyblogger is spot-on.

Some key points:

• Know who your reader is. Appeal to his or her fears and hopes. Tap into his or her beliefs and paint a picture of the world he or she wants to live in.
• In truth, it all boils down to this: eliminating anxiety.
• Love, Fear, Greed, and Duty are key emotions to speak to.
• Give the reader the sense that you will bring him peace (financial, future, relational, future, security) … that you’ll solve his problems that keep him up at night … that you will give him a good night’s sleep … and you will win his attention.