If you send out an e-mail newsletter (or even use e-mail for client communications), check out these Seven E-mail Landmines.  One tip that’s made me rethink my e-mails is this one:

The top 145-200 pixels of an email's height are the most critical. Key information to include above the fold:

• Company logo and link to the home page
• The main call to action, plus a link to act on that call
• A visual that enhances the brand image
• A headline that encourages readers to read the rest of the message

From a design perspective, the most common mistake is to clutter this section with graphics. If stripped out or blocked, they negatively affect your message.