No matter how much I write, and I write a lot, there are certain things I never seem to quite master. Proofreading my own work is one of those things. Why is this? Well, I know what I'm trying to say in my head, and thanks to some quirk, my brain superimposes what I think I've written over what I've actually written as I read. I've proofread copy a hundred times, only to publish it, go back months or years later and see some juvenile mistake.
This is why time is an important component in the writing process. Not the time you spend writing, but the time between when you finish the first draft, and when you start the revising process. The longer that period of time is, the better.
This isn't always possible. When I have a client who needs a 12-hour turnaround on a piece, obviously I can't wait 2 weeks to go back and peruse it.
But, dear writer, if you can, control your urge to revise immediately. Take a deep breath, enjoy what you have accomplished, and walk away. Go catch up on laundry, or re-watch LOST, maybe finish War and Peace.
This time kind of circumvents that little brain quirk, and you will forget what you wrote, and what you were trying to say. Two things should happen then, when you return to your manuscript:
You'll discover some golden passages that are better than you thought they would be.
You'll discover some not-so-golden passages that are not as good as you thought they were.
You'll see where your piece works and where it doesn't, and you'll get a better idea for flow.
Have you ever been up late reading a book and you realize you're close to the end and you kind of speed read the last 50 or so pages? I do that. And I write like that. When I get close to a key plot point or climax my prose become sparse indeed. Stepping away helps me to see that.
This is something I used to advise my students to do, and almost invariably they turned in work that had clearly been slapped together last minute. In the end they were only hurting themselves.
So, take a little time off when you finish a piece. You've earned it, and you'll view your work with fresh eyes when you return to it.