• People 'scan' email -  No need to tell you that people get a lot of email these days. Once an email is opened, it has approximately eight seconds to engage. Scanning is how we now decide to read, click or delete. In Jakob Nielsen's 2004 Email Newsletter Usability study he found only 11% of the participants read a newsletter completely and 10% put it aside to 'read later' - all other participants scanned. 
  
  
• Present content in a scannable format - Nielsen's survey also found the actual amount of content presented is a matter of perception. Users in the survey commented that one newsletter had "too much content", when actually it only contained 350 words. The text was presented in one large block - which made it appear longer and harder to read. The key is to break up text in your articles. This can be accomplished with space breaks, bullet points and bolded headers, all of which help the eye scan and absorb.
  
  In my seven years of experience in email marketing I have found the most successful newsletters have articles that are between 300 - 800 words long, 1,000 words maximum. Executive briefs or synopses leading to articles should stay within the 100 word range. A general consensus among marketers regarding a content mix that gives a feeling of value to the reader: 20% marketing material and 80% information.
  
  
• Too many choices dilutes your chance of a click - A recent EMEA DoubleClick trend report says the average current click through rates are now between 4 - 10% (based on emails delivered). This figure shows what a challenge it is to get subscribers - once they've opened - to then click. Too many articles can make the reader feel overwhelmed and say 'later', or miss the relevant article amongst the content 'noise'. For most newsletters I'd recommend 5-7 items and/or articles.
  
  
• Each item or article should have a Call to Action - Each content item should appear in the newsletter for a specific reason. Is it to highlight a product/service? Is it to upsell? Is it to get attendees to an event? Raise awareness of your service? Brand your company or showcase your expertise? If you are looking for a specific result, tell the reader what to do, e.g. 'call now', 'email now', 'book now'. (I've written about Calls to Action previously.)
  
  
• Don't use up all your content and ideas in the first few mailings - There is a natural tendency when first starting out in email publishing to put everything into the first few issues. There's a perception that more gives better value to the reader. Aside from the points already made as to why this isn't a good idea, a big one is you'll soon run out of content and ideas. To avoid this, set a publishing schedule for your company, plan ahead, plot out the articles and what issue they'll appear in. (You might find this interesting: Is your content worth reading?)

Addendum: Thanks to Mark Brownlow (Email Marketing Reports) for his additional suggestion - "There's always the option of asking readers what they'd prefer, too." This is very true. You can ask for direct feedback, or run a survey asking specific questions about content, length, frequency of newsletter, etc.