Write down feelings as well as ideas. Have a well-organized action plan you can follow automatically when an assignment comes. Don't allow interruptions when you are intensely at work. Teach your family or friends that your writing is not a simple hobby but a profession. Verify everything you write.
Editors are human.
Treat them as such. Once you have a foot in, you can break a few rules, negotiate contracts, etc. But when starting out or breaking into a new market, follow the rules and don't make waves. Meet editors, etc. in person whenever possible. Make contacts and use them.