Everyone loves a list, here's one with 17 basics for designing your email newsletter.

1. Ensure the From Address is the one you always use for your email marketing.
2. The Subject Line should make it clear that it is your newsletter (not a sales message/advert).
3. When opened, the newsletter should have a similar appearance to your marketing emails but it must be obvious that it is not one.
4. Use the same type of header that you use with a marketing email but change it subtly, perhaps a different colour.
5. Have your logo on prominent display.
6. Segment your subscriber list and tailor the email newsletter to suit the specific requirements of each group.
7. Have a brief introductory paragraph. Use it to introduce the newsletter rather than fluff about your company.
8. Use the form of address that will make the subscriber read on – "yo homie" probably isn't the best way to communicate with silver-surfers.
9. Do not list the entire contents of the newsletter. Make them read it all the way through and link further content back to your website. However, see (12) below.
10. If you have a series of articles, ensure there is one for each of your segmented groups.
11. Put first the article that that particular subscriber group will appreciate the most and give it a fulsome introduction, emphasising the points that will make them read it.
12. Put the article you feel the group would find next most interesting further down the list, perhaps third or fourth. This will make them read on. However, vary the locations. We don’t want regular subscribers skipping articles.
13. Do not give all the details relevant to the article at once. Have a click through to a landing page where you can also advertise your particular product/service associated with the email newsletter.
14. Limit the amount of copy. Just because you have sufficient articles to fill two newsletters should not mean that you make it twice as long. 
15. Have just the one call to action. Put it near to the main article for that particular subscriber group.
16. Do not slavishly follow all the previous 15 points. Experiment and see what is best for your company and subscribers.
17. Remember, subscribers rarely read lists longer than fifteen items 😉