As important and valuable as revision is, it's also true that you can overwork a story. At some point, revision has to end. So how do you know when it's time?
Lee K. Abbott says he knows his story is finished when he reads through it and can make no improvement except to delete the commas he added during the previous revision.
Jane Smiley says you know you're done when you're tired of the story: "You can't think of what else it needs. It no longer seems flexible to you-if you were to change some large piece of it, it would fall apart. It holds together. You know more about the material than you have put on paper, but if you added something, it would be a little repetitive. There is nothing you want to add."
So let's say that in three months or three years, you're finally finished with the story. You're really done. At last. Before you put it in the mail, there's one last step-polishing.
This will be the stage in which you go over the story with a fine-tooth comb, double-checking for any mistakes in spelling, punctuation, and usage. (For instance, I just hit the dictionary to make sure "double-checking" was hyphenated as a verb.) You should invest in a good dictionary and a copy of The Chicago Manual of Style (if you haven't already done so.)
This is also the stage where you'll do any last-minute fact-checking. When your protagonist chugs the soda on page three, is it a "Dr Pepper" or a "Dr. Pepper"? (It's "Dr" without the period.) When she drives from Cincinnati to Cleveland to see her dying father, did you use the correct interstate number? Are the details of the hospital room in Cleveland accurate and realistic?
An incorrect fact can shake an editor's faith in your story just as quickly as a clumsy beginning or cliché characterization. The same is true for misspellings and sloppy punctuation.
Granted, any editor worth her salt will be able to spot a gem in the rough, but with all the competition you're up against, why take the chance of your story being passed over because of cosmetic problems?