Most publishers are too generous and need to stop more readers [with a paywall] to force conversion. 

There's been an interesting discussion on the SIPA Forum this week—join here if you're not already on it—about how much content should be given away and what type of paywalls works best. Here are some interesting industry examples I found:

On Monday, the Christian Science Monitor launched Monitor Daily, a daily news digest of five pieces of content (stories, videos, graphics), plus one editorial and "one clearly labeled religious article offering spiritual insight often related to the news." It's emailed to subscribers each weekday at 6 p.m.

"An easy micropayment system for digital content could help save journalism... In my book, The Innovators, I report on how the creators of the web envisioned protocols that would allow digital payments, and I argue that this would benefit individual artists, writers, bloggers, game-makers, musicians, and entrepreneurs." —Walter Isaacson, best-selling author