Many writers feel exhilarated or relieved when they write the final word and want to get it to the editor or agent. Resist that urge. Look at it again critically. Does this sentence make sense? Did I explain it thoroughly? Did I over-explain?
After I close the file on a manuscript and leave it a few days, perhaps as long as a month, I've always improved it. I use the absolute always because I mean without exception.
When I return to the material, I read it with new insight because the material has been churning in my unconscious mind. (I intentionally put the previous sentence in the passive voice. I could have written: My unconscious mind churned the material, but the emphasis was on the action (churning) and not on the actor (my mind). This is an extra tip.
Write to get the story written; rewrite to improve the quality.