Background:

The reasons to support a PBS station by becoming a member are quite different if you're already a viewer but not a supporter than if you're not even a viewer. 

The approach:

So that we could speak directly to both audiences, we set up a double letter in an envelope that was the maximum width to still qualify for non-profit postage rates.  

The left side of the letter spoke to people who didn't already watch Thirteen.  It talked about the extraordinary regular series on Thirteen, in an effort to get people to watch.  It dispelled the myth that public television is just opera, deadly serious news, and drama with British accents.

The right side of the letter provided both logical and emotionally resonant reasons for why current viewers should support the station they were already watching.  It also dispelled the myths that (1) viewers pay for Thirteen when they paid their cable or satellite TV bill and (2) that Thirteen and PBS receive "big" tax dollars.  (The truth is that government support of PBS translates to about one dollar per person per year.)