It's a best practice to provide your email subscribers with an easy-to-access Email Preference Center and link to it from your footer. Be proactive, too. Encourage subscribers to visit the Center and update their content preferences, take a survey, sign up for more/different newsletters and update their email addresses.

Send a specific "Get More of What You Like" campaign every quarter, or include a promotion in the sidebar of your newsletter. Be sure your program keeps pace with your subscribers' lifestyles and life stages, to ensure relevancy and readership (and low spam complaints).