If you publish your ezine with weekly or greater frequency, consider offering a less frequent digest of your content. This works especially well for daily ezines, as the email load may be too great for some of your readers. By offering them a weekly, biweekly, or even monthly digest option, you're effectively keeping subscribers that might otherwise leave because of not being able to keep up with the amount of content you churn out. Related to the idea of digests is that of offering a summary of your content. Your subscribers might be overwhelmed by 3-5 specific newsletters per day, but a daily summary of the highlights of all those publications would be well received by those subscribers.
Summaries tend to work great as HTML-format ezines.
I'll cover HTML email publishing in detail in an upcoming Ezine-Tips. Finally, you could allow your subscribers to select to receive only specific kinds of your content. For example, they might choose to receive only the feature articles, or only the book or product reviews, or only articles on a specific topic. You could set up a list for each of these, or implement some type of database solution to manage all of this. More on database subscriber sorting in a future issue.