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Basically do it the same way you would make a web page. I think it's best to use tables for the layout. That's just my preference.
Make sure your images are small and compressed.
I usually set the width at 650px.
Keep in-mind that a lot of email clients don't show images. So I include a link at the top of the email that says, "Can't view the images? View (note: never use 'click here') the entire HTML version of this email on our website."
Avoid using words like "limited offer" or "save big".
Specifying Doc Type and Charsat will also help.
Test your email in all of the major email clients. Outlook, Yahoo!, Hotmail, AOL, etc...
I'm sure someone will add much more to this.