Maybe one of the hardest things as a writer is knowing when your story is finished. When you hit the save button for the last time, close the file, dust off your hands, and say that's it.
Listening to the last ISBW, Tim Pratt talked about how he knew people who would continue working for years on a story, always revising. Perhaps it got incrementally better on some level, he said, but you can always tweak things. I also remember reading a piece about a screenwriter who spent years getting the first page right. He was so proud, years later, feeling he'd nailed it. Now, he only had 119 to go.
Then there's the flipside. I've got a friend who writes an insane amount of stories but is horrible as rewriting and usually, I think, doesn't. He finishes a story, polishes it some, and sends it out. This is an extreme example, of course, but I think it's typical to have a lingering fear of, "Did I send it out too soon? Was there something else I could've done to make it better. What if..." Which can take you right back to the problem at the beginning of this post.
How do you know when you've finished a story? How do you balance the two problems above?