I've talked about Let's Code JavaScript on HN before; check out the previous thread [3] for some details about how I started and market the site.
Since simonswords82 was so generous with details about his business, I'll do the same. About 2% of the visitors to the site sign up for the free trial. That seems poor, although I don't have a lot of insight as to what the number "should" be. I do require a credit card in order to sign up for the trial. The trial automatically converts to a subscription after seven days.
About 70% of trial subscribers convert to a paid subscription. This seems pretty good to me, although I'd of course be happier if it was higher. Once they've subscribed, I lose about 25% of subscribers in the first month, and then the number drops asymptotically down to about 5% (of the initial total) per month. I'm reasonably happy with this as well. It's poor by the standards of a SaaS, where a churn rate of 1-2% is doable, but I think it's pretty good for a content site.
My most recent push was an inbound marketing campaign: I took one of my monthly specials and turned it into a dedicated site on object-oriented JavaScript [4]. That was very successful; it nearly doubled traffic in the first month (from 2,800 uniques in the previous month to 5,400) and looks like it will permanently add about 1,500 uniques per month. Trial rates went down slightly as a result, but overall subscriptions went up.
The biggest challenge for me is having lots of things I'd like to pursue, but not enough time to do them all. The Object Playground inbound marketing campaign was successful, but a lot of work. I recently released another special that might make a good inbound campaign (on large-scale JavaScript fundamentals: modularity and automated cross-browser unit testing), but I'm not sure it's worth the time it would take to create a marketing site for it on the scale of Object Playground.
For now I'm focusing on A/B testing various ideas for improving conversion. It takes less effort and potentially has longer-term impact. So far, I've improved the subscribe page [5] by adding testimonials and a better design. Next, I'll probably provide more opportunities for people to preview the series without signing up, as well as more obvious calls to action on the video pages. I also have some other plans in the works that I can't talk about yet. :-)
[1] My screencast is Let's Code: Test-Driven JavaScript, a series for professional JavaScript developers. It's available at http://www.letscodejavascript.com. I'd love to hear your feedback.
[2] My book is The Art of Agile Development. It's been out for over five years now, so it's nice to still be getting royalties from it.
[3] I talked about my experiences launching and marketing the screencast here: https://news.ycombinator.com/item?id=6038226
[4] My recent inbound marketing campaign was Object Playground, a tutorial and visualizer for object-oriented JavaScript: http://www.objectplayground.com.
[5] The Let's Code JavaScript subscribe page is at http://www.letscodejavascript.com/v3/subscribe.