I just read this great article about visual hierarchy and conversions. It's main focus is on web page layout and design and different techniques you can use to give areas of the page more visual dominance. You can easily take these concepts and adapt them for email marketing and increase your conversions and response rate.
The most important thing: what is your primary goal of the email? Think about what action you want the user to take or what the main goal is for your email message and structure the page content and design specifically around that goal, giving the place where the user should click more visual dominance to encourage that response.
What's great about this is that we as humans are hardwired to respond very predictably to certain visual queues, so marketers that understand these simple principles can drastically improve the response to their email marketing campaigns without spending any more money.
Read the full article here: http://searchengineland.com/pop-this-how-to-manage-visual-hierarchy-for-conversion-63053