This is a copy of an answer to an ask I got on tumblr about writing advice.
1) Read the kind of thing you want to write. If you want to write fanfic, read a whole lot of fanfic. If you want to write original mysteries, read original mysteries.
I know this is advice everyone gives, but I feel like what you are reading, even if you are not consciously trying to work out How the Author Did It, tunes your brain to a certain wavelength.
2) Read/listen to craft books, or follow writing tumblrs, or read articles about writing, or listen to podcasts. Anything really. The point isn’t so much specific knowledge as a background of information that you can slowly add to, and the bright points that will stick out and lodge in your brain, like OH YEAH THAT SOUNDS GREAT, I SHOULD TRY THAT. Or OH NO THAT SOUNDS TERRIBLE, I WILL NEVER DO THAT. (Caution: one can often become the other over time.)
3) If something isn’t working, try doing it another way. Your stories and your process are infinitely flexible. Something that used to work may no longer work. That’s cool. It doesn’t mean something’s wrong. It just means that you’ve changed as a writer or a person and that’s normal. There are a bunch of writing books out there that insist their way is the only way. That is total bullshit. Try lots of things. Try ALL the things.
I read something once a long time ago that has stuck with me (one of those bright points that lodged in my brain): when we get frustrated and feel like suddenly our work sucks balls, it’s not the work. It’s a shift in our perception or our goals. We’re reaching for something more, and so what we’ve got seems inadequate. Sort of a corollary to that Ira Glass thing, which I totally recommend if you haven’t seen it.
I hope some of that was helpful, and good luck. :)