Edit, edit, edit.
Your work will improve. You are bound to come across areas where you’ve repeated yourself. You don’t always come up with the right phrases first go. Cutting work tightens it. If you can get away with removing something without affecting your story, then you should. There should be nothing in your story that doesn’t need to be there.
When submitting work, always follow guidelines.
To do otherwise looks amateur. In competitions, it will get your story/novel automatically discounted. It shows editors/publishers you can follow instructions and pay attention to details.
Always include a stamped self addressed envelope (SSAE)
Again to do otherwise looks amateur. Work can be dismissed if an SSAE is not included. And you do want to get your work back or at the very least find out what happened to it, yes?
Always put on proper postage.
If in doubt check with the Post Office. I’ve occasionally had letters where I’ve had to pay the missing postage. It is very aggravating. Not the kind of impression/feeling you want to give to an agent/publisher!