I'll add one that's so obvious it's easy to miss:
Speaking as one who is living proof you can learn and improve SQL Server skills on the cheap, one of my greatest assets in addition to the excellent resources in the article is having my own SQL Server sandbox. I have a Developer Edition on my desktop at work that I can try out all kinds of things, including the Things-You-Should-Never-EVER-Try-On-A-Live-System. Most PCs these days are powerful enough to run it, and you can pick up a copy of SQL Server Developer 2012 for under $50 (no, that isn't a typo) at Amazon. Granted you're not going to be able to play with the entire gamut of SQL Server (mainly thinking of enterprise-wide architectures and the skills needed for those scenarios), but there's certainly enough to keep you entertained and learning. I can learn a great deal from reading and listening, but until I throw it on the hardware and say, "What happens when I do THIS?" does it really stick in my head.
Just my $0.02 from over here in the cheap seats of the peanut gallery - please adjust for inflation and/or your local currency.