According to Scott Palubinsky there are three main
aspects of email marketing: list quality, creative rendering, and
reporting:

1. The  quality of your list is much more important than the quantity.
   Growing your list is important, but what is more important is growing
   your list by adding opt-in subscribers/customers who will positively
   respond to your campaign. By segmenting and targeting the contacts on
   your lists, you’ll have a much greater chance of reaching your desired
   audience and getting a positive and profitable outcome.  Once you have
   your list developed, be sure to keep it clean by managing bounces and non-responders.

2. Creative rendering can make or break an  email campaign.  Follow these guidelines
   for designing for the preview pane and keeping the call to action above
   the fold.  Test what your emails will look like in multiple ISPs and
   mail readers.  A tool like our Sender Score  Monitor Program
   can help with this.  Look and establish where problems with your HTML
   are occurring and ensure that the creative design of your emails is
   engaging your subscribers and not getting lost in the shuffle.  Your
   creative work may look brilliant in one ISP, but could lose its design
   integrity in other ISPs, thus preventing your message from making an
   impact on those contacts/subscribers.

3. Analyzing
   and adapting to the information gathered from your email reports is the
   best and most efficient way to improve the success of your email
   campaigns. The best reporting shows you open, click-through,
   forward and opt-out rates, as well as bounce reports and deliverability
   reports by domain.  More advanced reporting can also provide you with conversion data.  Use this reporting data to
   measure the effectiveness of your campaigns and show you what can be
   improved upon next time. 

Paying attention to
these three truths can help to dramatically improve your email
marketing program. 

Check yourself now: 

• Do you have a high quality
  list of subscribers that have specifically opted-in to receive your
  emails and who respond to the messages you relay in your emails? 
• Do
  you have a compelling email creative that is designed for the preview
  pane and that renders well across various ISPs?
• Do you have access to a
  comprehensive reporting tool that you check on a regular basis and use
  to improve upon your email strategy?

Source: Blue Sky Factory