A few technical tips

1. Keep the width of your email to less than 550 pixels, this way the recipient won't need to scroll left and right
2. Keep total file size (including imagery) of your email to less than 60KB - even though most people have broadband - connection speeds still vary dramatically.
3. Don't include:
• Cascading Style Sheets - email clients are far less sophisticated than web browsers and an email with CSS style sheets will not display correctly in most email clients.
• Animated GIFs - these won't rotate in many email clients, so your reader will only see the first frame.
• Flash / ASP / JavaScript - all of these will result in your email getting blocked by firewalls / spam filters. Potentially they could all be used to run nasty scripts on a recipient's computer.
• Background images - Outlook 2007 and Lotus Notes will not display these