Over the years I have created hundreds of emails and sent out millions. The key to a good email campaign is A/B testing everything. Its hard to know exactly what will click with a user. Try different subjects, body text, headlines, call to actions and even sender names. In my experience the clearer the better and always make sure you have a clear opt out option so you don’t get flagged as spam by the ISP’s.
When I started working at Tequila \ Chiat \ Day I found that they were using a print production process to create emails (proffing printed Photoshop comps then coding). Sounds silly but they were cranking through tons of emails this way. I helped them create a new process that eliminated an entire round of proofing and brought the html coders into the design process earlier so they could catch designs that would not translate well to HTML.
For Aryty I helped build an in house email system that both monitors clicks conversions bounce rates and spam ranking. Having to worry about spoiling your IP address really makes a designer aware of the canned spam act and how to steer clear of it. Even while playing by all the rules I have been able to maintain successful email campaigns.