I was told this by a published writer, so it MUST be true.
When your MS is ready, send it to one agent/publisher at a time and not to 10 or more. If the agent/publisher rejects it you can see if anything needs changing before sending it onto the next one. If you send it to ten at once you've effectively shot your bolt with ten agents/publishers in foul swoop.
Presumably the same applies to short stories.