I was quite fortunate to have an AP English teacher my senior year of high school that understood this advice, and expounded on it often. I recall that he boiled it down to one simple line:

Garbage in, garbage out.

What he meant by that was that it's not enough to simply read. You have to read high quality literature if you want to become a writer. You can't expect to read nothing but news, comic books, National Lampoon, MAD magazine (and in our era, blogs) and become a good writer. It just doesn't work that way. It's like trying to get physically healthy by eating junk food.