For writers of all sorts, the best piece of advice anyone can give is to read. Read, read, and read some more. Because if you don't read, how do you know what you like? Another great bit of advice you often hear as a writer is to write what you would want to read. And if you don't read? The obvious conclusion: you don't write.
A big part of reading, especially as a writer, is reading like a writer. When you read, you don't just dive into the story and submerge yourself into the characters and put yourself in the setting, though that's still important. No, as a writer, you've got to analyze. Analyze the style the author uses in certain situations, their overall voice, how they meld different styles together. Analyze how they establish the setting, how they keep you grounded in the reality they're creating, how they drive home the emotion of the locations. Analyze the characters themselves, how the author brings them to life, how they react to each other, how they act under duress, how they talk. You have to analyze how the author uses all of this to their advantage in the actual plot of the story, how they string everything together, where the beginning and end are, the pacing between each major event, the impact the climax and resolution really have upon the story. All of these things are majorly important as the writer reads anything.
The real problem begins, then, in deciding what stories to read? Many many people will say to read anything and everything you can get your hands on--which is true to an extent. However, I would like to argue that there are enough books published that if that you can easily get your hands in anything you want. The real issue at hand is choosing which books are more worth reading than others.
You can't just read bestsellers and award winners. Sure, they're the most popular. Sure, they're the ones people read the most. Logically, yes, if you write like the bestselling authors do, you'll sell all of your books to big publishing companies and write bestsellers yourself. But that's not how it works. Just because you write like a bestselling author doesn't make you one. There are too many emulating writers out there who have learned how disappointing it is when their book doesn't end up going very far.
In the same vein of thought, you obviously can't just read the bad stuff. You could argue that you're reading so you know what not to do--which is smart, don't get me wrong--but that doesn't justify reading only the negative end of writing. As a writer, when you read (or watch T.V. or movies, but that's a different story altogether) you're writing is influenced by that. When you read certain types of writing, your own is greatly affected. Therefore, even if you read the bad stuff, constantly reminding yourself that it's 'what not to do,' but have no quality reading to balance it out, your writing will be affected. You definitely won't end up going anywhere in that case.
Another thing you should avoid is limiting your reading. Don't only read your genre. As in, just because you want to write military science fiction (space marines, yeah!) doesn't mean you should just read military science fiction. Read other science fiction, read other speculative fiction, read other fiction, read some non-fiction. If you only read what's in your genre, then your only influences are things that have been done before. People notice when you write like someone else in the same genre because, chances are, that's what they usually read. So they know.
The question arises, then: what do you read? And, in light of what I've just discussed, you should read everything. Do read the bestsellers. Do read the bad writing. Do read what's in your genre. But know what you're reading before you start; and never limit yourself. Read and then analyze. When you read a book, stretch it. Get everything you possibly can out of it. Wring it dry for all of the precious knowledge you can get from it.