Neil Berman lists these 8 Steps To Good Mobile Email Design in his recent Email Insider column:

1. Send your emails in MIME Multipart Format so they are readable on mobile devices that can display only the text version of your email.  
2. Keep subject lines to 15 or fewer characters or front-load it so your key message is in the first 15 characters.  
3. Make the most of pre-header text. iPhones allow about 140 characters in vertical view, so keep the text short and persuasive. 
4. For the best display on both mobile devices and the desktop, code your emails between 480 and 600 pixels.  
5. Keep the layout simple and stack information in one column. 
6. Take it easy on the images. 
7. Code all links and buttons with a target area of at least 44 x 44 pixels.  
8. Test your design regularly on various devices to make sure your emails continue to render as you intended.

Source: Mediapost