Now and again I see someone on a message board grandly proclaim "I prefer to focus on the quality of my writing rather than the quantity." This is a comment usually pointed at someone else who is celebrating having finished a draft of a novel in a month or written 10k words in one day or otherwise having worked hard and fast.
First let me say, we should all think about the quality of our writing. But I think it's rather obvious that no one intentionally writes poorly and the vast majority of us seek to improve our skill set rather than maintain whatever level we started at.
Now. Here's the plain fact of the quality vs quantity myth.
You will not get better at writing if you aren't actually writing. This means writing every day. This means setting challenging goals. This means producing fiction (or non-fiction if that's the way you swing) in quantity. The more you produce, the better you will get. End of story.
Writing is how you get better at writing.
Of course, there are things you can do to help improve the quality of your writing. Identifying flaw/weaknesses and trying to eliminate them as you write is always a good idea. (I would suggest only focusing on one or two at a time though, otherwise you'll be overwhelmed.) Reading books that are well-written and figuring out why you like them is a good idea. Reading books that are poorly-written and figuring out why you don't like them is a good idea. Making notes on words you aren't familiar with, pulling down the dictionary to see if that word you've never used before really means what you think, is a good idea.
But the best idea of all, is to write.
As much as possible.
Write. Write. Write.
Because quality comes from quantity. Not the other way around.