Read your stuff out loud.
I’d even go so far as to say that if you can stand the sound of your own voice (I can’t — not yours, dear, but mine) read your work into a recorder. Play it back.
I recommend this because writers don’t change much between 5th grade and 30, I find. When told to read something aloud, a writer starts happily and everything is good until suddenly:
“And he walked down the street with a…fish…in his pocket. A fish in his pocket? That’s not right! What the hell does that even mean? What was I thinking?!”
Or (better yet):
“I can’t even read what I wrote here. Sorry.”
Hide under your desk and do it. Close your door. Catch those little things before your editor does (or worse, your mom). Actually forcing the words out of your mouth gives you an idea what they are going to sound like to others. The practice slows you down, gives you real focus on the ebb and flow of your nouns, verbs, consonants and vowels. You’ll realize that what you thought was a great scene of dialogue sounds like two people trying to have a fight in another language.
Like a symphony, it can look fantastic on paper, but if it isn’t beautiful to hear, you’ve lost.