One mistaken assumption that I've noticed some newbie writers making: Sending out their writing too soon, assuming that the editor who buys their short story (or novel, etc.) is going to be helping them polish the piece anyway.
DO NOT DO THIS.
Never, ever send an mss out just after you've finished it. Put it away for a few days (a few weeks at least, for a novel). That way you'll be able to reread more objectively, without the rosy glow of "omigosh this is brilliant just wait until publishers see this."
I'm a foodie, so often think in terms of food analogies. In this case, it would be sort of like a first-time restauranteur opening before they've perfected their dishes. Turn off the restaurant critics early on, and you make it tougher for yourself longterm.
If you're a new picture book writer, this is even MORE vital. Why? Because I've noticed that many non-pb writers assume that writing a picture book is easy because there are fewer words, that it's something they can do on the side for extra money while they work on their "real" books.
Vaguely related side note:
Others may differ, but I also advise NOT giving it to your critique group to read too soon. Why? Because there is a real value in getting feedback from someone who is reading the piece for the first time. Yes, there's a value in getting feedback for a rough version so you can polish it before sending it out to an editor. Be aware, however, that after the first critique, your crit partners will likely be giving feedback on your revisions rather than an overall first-time impression.
Respect your readers, before and after publication.