This Shorenstein Centre whitepaper has a bunch of tactics, harvested from over 500 for-profit organisations, that publishers can use to increase and retain digital subscriptions.
Key quote? "Across the news organizations studied, we found that subscribers who also receive a publisher’s email news product are 5-10 times more likely to subscribe. Those who follow the publisher’s brand on social media are 4-6 times more likely to subscribe."
Key takeaway? Experimenting with price and customer targeting can radically improve retention rates.