some more:
unit testing - annoyingly rather handy, had a good argument about why it's better not to use it, well I use it, I lost that argument but gained a lot.
design patterns
open source projects - a great benefit to your skills, there's something that needs kicking for any level.
learn why it's handy to use version control rather then just use it (tip - it's because if something borks can roll it back a lot easier)
linux - not just learn it, know it. bash scripting, piping commands and expect all have saved me multiple hours
rss - handy