The Messaging Times offers 3 great tips to improve your email marketing campaigns in 2008. Here they are:

1. Create a calendar for your email marketing campaigns so that you have a visual representation of your recipients, content and frequency (Who, What, When). Paste it on the wall in front of your desk or save it as your computer’s background image.
2. Design an email marketing checklist and use it every time you send a campaign. It’s even more important to use a checklist after you’ve been doing it for a long time. Attention to detail is the cornerstone of successful email marketing.
3. Test your campaigns against as many email clients as you can prior to sending (include testing on your email marketing calendar). You can either establish accounts yourself with Outlook, Outlook Express, Thunderbird, Hotmail, Yahoo, Gmail, etc. or you can use an email design testing service and have them test your message for you.

I'd like to add a fourth one:

4. Create an editorial calendar that contains an overview of the content for each email that you will send this quarter/year. That way you will not be searching for content at the very last minute.