There are many ways to keep in touch with your audience. In spite of emerging channels such as social media and mobile device platforms, email marketing continues to remain a strong performer.

On the technical side, designing a rich email newsletter can be a challenge, due to the wide variety of email clients used by recipients of your newsletter. When a person receives your HTML email, their email client is acting like a browser to display the page. The problem is - these browsers are much less capable than "real" browsers, and vary widely in rendering capabilities. By and large, you've got to design to the lowest common denominator, and make sure your email is displayed properly in the "worst" email client.

How bad is the "worst" client? Think 1998. Roll back your technology 10 years and then you'll be in the neighborhood. Tables. Inline CSS. Its not pretty on the inside, but you can still get the job done with a little guidance.

Firstly, if you are considering using email campaigns with any frequency, I would recommend a delivery service. Five years ago, you'd have had a decent chance of being able to send bulk emails from your own computer, depending on your ISP's mail policy. SPAM has changed this landscape quite dramatically in the last few years, and it is nearly impossible to send emails yourself these days. And with a growing number of easy-to-use campaign services, it is a no-brainer:

Mailchimp

Campaign Monitor

iContact

Constant Contact

Emma

Mad Mimi

The above sites include a wealth of information about the entire process. Mailchimp has an excellent guidebook that is quite comprehensive without being overwhelming. Campaign Monitor has a well-written set of articles with some of the same topics.

Consider using a template provided by the service you choose. Campaign Monitor has a nice selection of 30 templates while Mailchimp gives you four flexible starter templates to base your own design on.

A nice article at Sitepoint gives a comprehensive guide on safe coding techniques.

Now that you've mastered email design and delivery, you can contribute to the effort to make all of this easier (and better for the web!) at the Email Standards Project, an organization dedicated to improving support for web standards in mail clients.