One way to do it is to use a template. There are free ones that have been tested (or so they advertise) and some you pay for (I used to use one from High Impact). For free ones you might want to check out:
http://www.mailchimp.com/resources/templates/
Personally, I have an email newsletter now for which I just decided to send out a link in a brief simple html message that is no more than a few short paragraphs promoting what is in the newsletter and a link for the full contents.