This one is an oldie but goodie:
Writing is a business. Act like it.
“Every writer who writes for pay is running a small business. You have to create product, track inventory, bid on work, negotiate contracts, pay creditors, make sure you get paid and deal with taxes. Work has to be done on time and to specification. Your business reputation will help you get work — or will make sure you don’t get any more. This is your job. This is your business.”
—Unasked-For Advice to New Writers About Money by John Scalzi
Traditional writers (or traditionally minded writers) might be less inclined to treat their art as a business. This might be because it is the job of the agent, the editor, and the publisher to take care of the business side of things.
The world of self-publishing, on the other hand, is very driven by authors that spend as much energy in writing as they do promoting. Many of those independent authors plan the business side of writing, even before they write the first sentence of their books.
In summary, act like a pro, if you want to be one.