I just attended a pretty good webinar produced by Target Marketing Group called "Design that Clicks: Crafting Emails that Encourage Greater Response". You can view the entire webinar for free here (in exchange, of coarse, for being subjected to some harmless plugs). Some of the takeaways were:

+ Design layouts in Z reading patterns similar to the eye movement graph above
+ Develop ongoing dialog with customers
+ Give immediate reason/benefit to clicking thru
+ Provide many links/opportunities to click thru
+ Long copy typically doesn't work
+ Use personalization (text or image) in subject line or above the fold
+ Test, test, test: use an A/B split to test subject line and copy
+ Although it's commonplace in China, sending Flash based emails in North America only gives you a 50/50 chance of deliverability
+ Definition of BAC'N: email you want, just not right now

and the #1 takeaway...

+ Be genuine and respect your audience. Not only by providing a privacy policy and an ability to opt out, but through your language choice. Loud copy like "Hurry! Limited quantities left!" could insult their intelligence. Learn who your audience is and speak to them in a tone they can relate to.