Campaign monitor guidelines, posted end of 2005. Very good as general rules:
1. Never use images for important content like headlines, links and any calls to action.
2. Use alt text for all images for a better experience in Gmail and always add the height and width to the image to ensure that the blank placeholder image doesn’t throw your design out.
3. Add a text-based link to a web version of your design at the top of your email.
4. Ensure your most compelling content is at the top (and preferably to the left).
5. Test your design in a preview pane, full screen and with images turned on and off before you send it.
6. Ask your subscriber to add your From address to their address book at every opportunity.
The complete list with explanation.