2. Effectively Use Subject Lines With Pre-Headers: A pre-header is basic text that can be found at the very top of your email. There are two schools of thought regarding pre-headers and mobile devices, both of which should be tested. The first is that handheld devices make the use of pre-headers a necessity, because it's prime real estate where you want to include key information and offer specific details that entice the consumer to read on. Some marketers use text phrases (i.e. "click here if you cannot see images" or "add us to your safe senders list"). However, because this is likely the first thing a prospect reads, it makes sense to highlight your offer or promotion.
The second theory is to eliminate the pre-header text altogether, because it pushes the email creative down the screen and less of the email is visible.
The best advice and compromise is to keep pre-header text to one or two short lines, so the email creative is visible while still highlighting your best offer copy. Ensure that the subject line and pre-header text are working to support each other, and keep the subject line somewhere between 30 characters to 45 characters for mobile devices.
[Side note on wearables: While information on the new Apple Watch email client is still fairly thin, early predictions show that subject lines and pre-header text will be more important and relevant than ever. Even The New York Times announced it would begin publishing one-sentence stories and other content on various topics for the Apple Watch, enticing users to "hand off" or read a summary of a story or email on the Watch, then use another device to read the full article or respond to an email.]
3. Use a Single-Column Ladder Design: Sidebars and two-column designs are the gold standard for desktop creative, because you have two separate areas to feature your best offer copy so that it appears at the top of the email preview pane where it's more likely to be noticed. This is known as the "above-the-fold" area, and is commonly where your headline appears. However, a mobile device will likely shift the columns, text and images, and create overlapping areas, which create a poor presentation. The single-column design keeps it simple, with no rendering issues to contend with, no columns being pushed into text and images aligned as you intended.