How to be a Better Writer
READ.
That is all.
Ok. I'll write more. But seriously, that's what it boils down to. Good readers become good writers. It's the same principle as nutrition, if your body is what you eat, then your mind is what you read.
Consequently, that means you have to read GOOD books. If all you read is Twilighty crap, then your writing will remain juvenile and undeveloped. But if you are constantly stretching yourself and pushing yourself to read more challenging texts, and then pushing yourself to articulate things that are more and more complicated, then you grow stronger and more fluent as a writer.
After reading, I would say practice, practice, practice. I've been writing various stuff for 18 years now and while I'm paid to edit and assist (and sometimes tutor) people in writing, I would not consider myself to be a good writer. The English language is so complex, so varied and nuanced, and human experience is so far beyond the constructs of language, that to assume you're a good writer is FOLLY. Practice writing what interests you--stories or essays or poetry--and never allow yourself to get complacent, there lies stagnation and death.
As you're practicing you have to find your "voice." It's a hard thing. That's why reading is brilliant, you read other people's voices to better hear your own. And then you have to learn to trust it. Don't compare it to others, don't try to be like other writers you might admire, be yourself. Write with your own voice, and let that voice guide you. And while you may never write the Great American Novel, you will write things that are TRUE to you. And because they are true and YOU, they will be great in their own manner.