Here's an article I don't want to lose track of: How to Create Great HTML Emails with CSS
To quote only the summary...
General Do's and Don't's for HTML/CSS Emails
To wrap up, here are some do's and don't's:
* Use inline style declarations for the most important styles.
* Declare styles in a style tag when they're not absolutely necessary.
* Use table tags for multi-column layouts and floating elements.
* Test your email in multiple standalone and Web-based email clients.
* Use external stylesheets.
* Use floating div tags for multi-column layouts.
* Expect CSS background images to show up in most email clients.
* Rely on images showing up, especially background images.