CheetahMail offers a number of excellent whitepapers on their website. One of these is called "Creative Best Practices". Here are a few tips they offer:

Preview Pane Area
Outlook’s preview pane window is a commonly overlooked element in email designs. The designer only has at the most, 300 pixels of height to present the email’s main content and navigation to an Outlook user. In this case, since the user doesn’t have to open the email to preview its content, this section should communicate the main message of the email quickly and efficiently.

Above-the-Fold Area
The area that appears above the browser’s bottom window bar is called
the "Above-the-Fold" area. This is especially important for
browser-based email clients such as Hotmail and Yahoo!. The
"Above-the-Fold" section on a 17" monitor with a resolution of 800 x
600 is 420 pixels in height. In most cases, these extra 120 pixels of space are used for content that supports the preview pane area design.
It is important to note that this area should be visually engaging as
well, since this is the first thing Hotmail and Yahoo! users will see
when they open the email.

Image Usage

• Never have important content appear only in an image. If
  that image fails to load, the main message of the email will be lost.
  This is important since more and more email clients like Gmail won�t
  display images in an email unless the user takes action to do so.
• Also, avoid spaces in your HTML code between an <img>
  tag and </td> tag. Some browsers will read this space as an
  actual space in the overall layout which can lead to misaligned
  graphics.

Importing Text Content from MS Word
Importing text content from Microsoft Word can lead to a few minor
problems all stemming from Microsoft Word’s non-ASCII characters. While
they may appear okay in Internet Explorer, Netscape users will see
strange letters. Be especially careful with apostrophes, double quotes and dashes!
With its default setup, MS Word will replace apostrophes and double
quotes with special characters not supported by many email clients.

The white paper includes a lot more interesting tips like:

• Email Client Deliverability (Gmail, Yahoo!, MSN & Hotmail, Lotus Notes, Netscape, Internet Explorer & AOL)
• Text Email Guidelines (look & feel, width & length of text emails, urls)
• Creative (call-to-action, email file size & physical dimensions, stylesheets, navigation)
• Technical (JavaScript, forms, background tags & colors,
  troubleshooting HTML, header tags, rowspans & colspans, alt tags)

Request your copy of this white paper here.