I actually used Illustrator to do the initial layout, which I then recreate in DreamWeaver (insert your favorite html editor here). The links Jack gave you are fantastic - you should be able to get a bunch of ideas from them. A couple of points to add:
1) Cross-test in multiple browsers. Outlook won't render CSS, so you'll have to do all your layout in tables (I know, I know - you would think that there would be some kind of "Standard" that all "Web" browsers could adhere to...). I learned this the hard way when, after spending significant time getting everything to work in CSS (and thus be much easier in future editions) only to discover that my beautiful, standards-compliant newsletter wouldn't display properly in one of the most popular mail readers. Looked fantastic in thunderbird and various webmail clients, just not Outlook...
2) Assume your users will be lazy and not use the "view in a browser" button. Then see point #1. As much as it sucks, we have to face the fact that, despite having beautiful and visually alluring graphics, our clients probably won't see them if they have to actually do anything beyond clicking on the email.
3) Get used to a tall, thin format. MailChimp (which has an amazing free package) and other services are pretty good about reminding you about this, but most mail readers' preview pane isn't much wider than that, and most folks won't click the "click to view in your browser" button (see point 2).
4) Insert standard boilerplate about putting the important stuff above the fold. That being said, clearly you can't get everything above the fold unless you're really cutting to the bone. A couple of ways to deal with this: a) don't even try to put the full article in the newsletter - just the beginning and a jump to the main article at the end and b) toss in a "headline" section at the top with links to named anchors so your readers can click from above the fold and be taken down to their article of choice.