Robert Heinlein's career began during what is known as the golden age of science fiction. While this meant lots of voracious readers with a big appetite for MORE, it happened during the Great Depression. This meant that magazines couldn't pay much per word, so Heinlein and his colleagues needed to get more work done in less time. He didn't have the luxury of carefully choosing each and every golden word that went into his books. He did the work, did it well, and moved on to the next piece. 
He often spoke of those times of quick writing and publication as a great learning experience. Authors were forced to learn how to write well from the start, and to produce on a regular schedule. He talked about his five favorite rules of writing, those rules he learned by writing hundreds of books and short stories during his career. He may have developed these rules decades ago, but they're still just as relevant today.
Rule One: You Must Write
It sounds obvious, but this first step is one that many authors fall down on. Sure, it makes sense to take classes and study plotting books and join author focus groups, but sooner or later you have to actually write. Every day. You can't just write when you feel inspired or when the muse hits you, but on a regular schedule like any other job. Don't complain about not having time to write, make the time. Stop watching television, get up an hour early, do whatever it takes to make sure you write every day. The name of the game is productivity, before any other considerations.
Rule Two: Finish What You Start
Having a hard drive full of unfinished novels does you no good. There's something to be said for finally finishing a book, even if it's the worst first draft on the planet. Every first draft is bad, that's why they're only the first version. The only way you'll create a good final book is to finish that bad first try, then edit it until it's better.
Rule Three: You Must Refrain From Rewriting, Except to Editorial Order
No, that doesn't mean you should leave the typos and plot holes in when you send your work off to the editor. What Heinlein did mean was to stop fiddling around with your work, trying to polish each sentence into a perfect string of words. Readers want to read your story, not your beautifully crafted sentences. Do your best work, fix the typos and plot problems to make sure it makes sense, then send it off to be edited.
Rule Four: You Must Put Your Story on the Market
In Heinlein's day, this mean mailing off the work to land on someone's slush pile in New York. Today, it can mean sending your work to an agent or, increasingly, self publishing it online. Too many people hang on to a finished manuscript, stuck with the butterflies in their stomach and unsure when to push that publish button. The first time is the hardest, but it really is only one small step in a series of steps to take on the way to becoming a working author.
Rule Five: You Must Keep it on the Market Until it is Sold
In the world of the slush pile, this meant getting your work back rejected, then sending it back out into the world aimed toward another magazine or publisher. Today, it means coming up with another book marketing plan, re-thinking your business model, or looking at your niche to make sure it's the right one for you. The idea is to keep writing and keep working toward getting your books in front of readers. The only way to fail is to give up.
Keep the Faith and May the Force be with You!
