Make your work as good as it can be before submitting it anywhere. But equally don’t use that as an excuse never to send work out. Ultimately submitting work is the way to get the feedback you need. If it’s really good someone somewhere will want that piece. You’re looking to make sure that nobody can read your work and automatically dismiss it. You want them to read it, love it, remember it and publish it!
Keep writing even if it is only short pieces, journal entries, blogging etc. See it as flexing your writing muscles ready for tackling longer works later on. When work is rejected, can you rework it and send it somewhere else? Can you turn a script into a short story or vice versa?
Books must survive. Not only it is it a sign of civilization to read and to have reading places, records must be kept. You can’t beat the book for that. DVDs go out of date etc, you have to choose new formats depending on how and when technology dates. You don’t need to worry about that with a book.
Expect rejections. Expect a lot of them too! If you’re given any specific comments consider them carefully. And just because one story has been turned down for one outlet, it doesn’t mean it’ll be rejected for all of them. Sometimes rejections can be for reasons outside of your control like the outlet you target has already had stories in on your theme. If, however, you keep getting rejections on the same piece, put that work away for a while, study it again later and see if you can spot weaknesses then. A time delay like this can suddenly open your eyes to faults. And of course you’ll be working on short stories all the time so you’ve always got something to send out or out there doing the rounds!