Reading. And not just a couple of genres, I mean as much as possible on as many topics as you can deem interesting. Classical literature, epics, poetry, contemporary criticism, post-modernism, every pretentious piece of work you can get your hands on.

Because the greater your vocabulary and the wider your understanding, the more efficient and proficient you become in learning new things.

Also, it makes you a better writer when you finally find yourself needing to put together some technical documentation for that content management system you whipped together in a fortnight.