1. Use a static IP address.
  2. Make sure your DNS entry is complete and correct. e.g. reverse DNS
    Tool: http://www.dnsreport.com
  3. Publish an SPF record for your domain
    Tool: http://www.spf.pobox.com/wizard.html
  4. See unsubscribes as a good thing.
  5. Keep an eye on delivery rates at individual ISPs
  6. Test your content. e.g. URL, words/phrases (FREE, Gurantee), excess formatting, images and attachments
  7. Use a professional email marketing solution. e.g. Spread
  8. Respect the recipient.
  9. List maintenance.
  10. Reputation:
    1. http://www.isipp.com/iadb.php
    2. http://www.bondedsender.com
    3. http://www.habeas.com
  11. Relationships & Whitelisting e.g. AOL, Yahoo