The most valuable tool any writer can have is a friend or colleague who is willing to act as a second pair of eyes.
When you've been working on a writing piece for a while, it's easy to become blind to errors in your writing. You start to miss obvious typos and examples of poor phrasing. Holes in your argument that would be obvious to another reader are invisible to you. And working longer or harder only increases your error blindness.
This is the time to bring in another pair of eyes. Find another competent writer who is willing to exchange editing skills with you, and read, edit and critique each other's work.
This can be an extremely powerful partnership with the right person. One of the most effective editing partnerships of my life was with my college roommate. We would just tear into each other's papers, often leaving sarcastic, even savage, comments in the margins. Our mutual editing process completely exterminated typos, grammatical errors and awkward sentences from our work. It was all in good fun, and we made each other's writing far better than we could have done on our own.
And I never got a grade below an A on any paper I wrote that year.