Short answer, read. Not just often and plenty, but read like every word can be used. Treat everything as a source, and you'll be using the world's secrets against it.
The slightly longer answer is: read what? If you ask me, I say look backwards. Look for the forgotten and trodden. The foreign authors currently out of print, tales found on clay tablets. Read poetry, drama, short fiction, novels, mathematical theories, scientific theories, story telling theories. The past is there for you to take and make work for you. It's good for that, seeing as it's solid. It's a sturdy foundation to build from.
But why should you read everything, if you're only interested in poetry for instance? What benefit is there in reading Mathematics and the Imagination by Edward Kasner? For starters, poetry hasn't said all that needs to be said about the world, and they never will. William Faulkner said that poetry was the most demanding form of literature. Roberto Bolano had similar feelings about the form, saying that "poetry makes me blush less."
So then, it's settled, arguably the best writers of American and Chilean literature have settled it; we should all aspire to poetry. But the thing is, these guys read vivaciously. Faulkner even expected the same of his readers. Those who do not understand simply haven't arrived. Those who will never understand are indolent.
Writing, I think, isn't something that can be taught in the classroom. You can teach the technicalities of course, but specifically, in fiction, the journey is singular and the closest your journey ever gets physical form is the work it creates. That is to say, it doesn't unveil itself very much at all.
I can only speak for myself when I talk about learning because it's also a singular, inward prophet's-walk. For me, I have learned more about life and the world from books and listening. Notice that, for me at least, the two greatest avenues of knowledge, for me, are done in silence.
Nothing in life is ever explained away by one source. Problems and solutions are often the cause or direct reaction to a bunch of different choices and events. Writing is kind of like life in a way then, because its source comes from all over. Math-inspired Borges to create his infinite library in The Library of Babel. Science gave Italo Calvino the tools to tell the phantasmic creation of the universe from the perspective of the universe, in The Complete Cosmicomics. Had they just been reading what they thought they were interested in or what was relevant to them as aspiring writers, ironically, they wouldn't have become the writers that they were.
Make books work for you. They serve you in a lot of ways. They might give you pleasure, grief, hatred. But they also provide you with another source which you can use to improve your writing. Analyzing their structure, how they present and back up their arguments (in the case of essays or fictional essays), can help you convince the reader of something. Analyzing how the characters talk, teaches you something about dialogue: did the author have an ear for it? Or does it come off stilted and caught in its carefully constructed sphere made by the author? Analyze how authors treat their audience. Do they explain every little thing or do they let the actions speak for themselves? Have they fallen too deep on either side of "show don't tell" or "for God's sake, tell, I got things to do today!" Analyze, most of all, the images they convey. What possibilities and feelings are forced into your head.
"A bee stung Achilles' ankle." What does that do to you? Nothing? Everything? How can you make it yours?
Mathematician Edward Kasner said "Poets in every age may have sung of the stars as infinite in number, when all they saw was, perhaps, three thousand." What's interesting here is that the poet's took the time to look up from their papers. They decided there was something they could not find in themselves, but which was reflected, all around fragments of themselves. These slivers of themselves we find in their books. These poets are hundreds of years old. They haven't felt the Greco-Roman "second death" where their works are forgotten. But, seek the forgotten, revive them and they'll pay it back hopefully, by making you wiser or at least more aware. This awareness builds, as the words of those who came before build up into a defiantly beautiful trash heap of understanding. Then you'll look up from those pages, confused that you hadn't seen a thing in a long time. And you'll look to the things around you for infinite inspiration.