Remember that visitors of a certain site are there because they are interested in what the site has to offer. If they sign up for an opt-in list or for a newsletter this means that they want to be updated for that certain theme or subject. Be sure that when you publish your newsletter you are providing for the need of the subscriber as well as their interests.
Ensure that you have well written, information riddled and content rich articles. You articles will be the body of your newsletter and that they should be able to excite your readers as well as provide information. Articles should be well written and checked for errors such as spelling and grammatical errors for it to look professional and believable. The trust of your client to you and newsletter is at stake here.