Read a book
Not exactly a novel idea, but there are some great books out there that will help you get up to speed.
First part Object Oriented JS, second part JS design patterns. It helped me think more about performance and memory usage in JS, even if a lot of the concepts will be taken care of by whichever JS libraries you use.
John Resig, creator of jQuery, digs into the skills necessary to write your own JS library. For the JS journeyman looking to be a master.
Go to a conference, or watch the aftermath
JSConf/NodeConf (over 100 videos since 2009)
Sencha (videos from SenchaCon and meetups)
Go to a local meetup
There’s no better way to learn JS than to learn it with some local people interested in the same thing. Check out these places to find a meetup near you.
beer.js – Around the world, JS geeks love to gather, drink, and occasionally code.
meetup.com – Join a group near you, or start one
Follow the Tweets
If you want to stay in the loop, keep track of what these guys are up to. This is who I follow, definitely not comprehensive.
Have more suggestions? Let us know!