Ask HN: How do you learn a new language or library?

I've been reading through the Backbone.js source code this week. While it's been a good exercise for my javascript-fu, it does feel pretty dull at times. I'm forcing myself to power through all of it because I know that I can't really form ideas with tools I'm unfamiliar with. So, out of curiosity, how do you learn a new language or library? What's been most effective? Has that changed over time?