In order to stay current in a technology you have to immerse yourself in community content.
Documentation is good for knowing the specification of a language or tool and books or training courses are great for when you want to dive deep on a particular topic. However in many technology fields, including SQL Server, one of the best ways to stay up to date is by following blogs.
Not only are blogs posts typically showing newer features and techniques that haven't been able to make their way into books and training courses yet, they often will cover more specialized scenarios and edge cases than you'll encounter in the traditional learning sources. Following a broad user base of SQL Server professionals who blog solutions to problems they encounter and will often open your eyes to features of the tools you use everyday that you weren't even aware of.
This is especially nice for times when I'm not actively learning a specific topic. Reading a variety of blogs on various SQL Server subjects often reminds me how much I don't know and how many features I don't use during my day-to-day work. These blogs also serve as inspiration to investigate ideas further.
My favorite way to follow other SQL Server blogs is by subscribing to RSS feeds. I use Feedly as my RSS feed reader of choice (RIP Google Reader) and browse through all of my feeds whenever I have downtime. Inspired by Brent Ozar's post last year, I'm sharing all of the RSS feeds I follow in this OPML file.
You can download the OPML file and import it into your RSS reader as a starting point, or go through the list of websites and see if you find something new. I decided to also include my non-SQL feeds in the above link too (they are categorized in appropriate sections) in case you are curious about what sites I follow for other programming and career topics.
It's impossible to stay up to date with all the information that I want to, but staying up to date on RSS feeds gets me close.
I'm always looking for more sources - RSS is so easy to consume - so if you have any suggestions please let me know!