I keep swearing I'll never mention politics (especially Scottish politics) again and then fall off the wagon. So visualize me climbing back on the wagon and getting back to the topic of writing.
I was going on a bit about Heinlein's Rules and his first two were pretty straightforward. You write and you finish what you're writing. But his rule three tends to make a lot of people choke. The one where he said to only revise to editorial order.
I suspect that he meant that pretty literally considering what I've heard about Heinlein. I once was told that when another well-known author complained about having to edit, Heinlein remarked, "Why didn't you do it right the first time?" I'm not sure if the story is true--but it seems right for Mr. Heinlein.
However, most of us don't get it right the first time. At the least, we have to do some clean-up, and I assume that he wasn't referring to that. I try to get it down in a fairly coherent manner the first time though and I think that may have been what he was getting at. We don't improve our writing, I suspect, without working hard at that first draft to get it as close to a final as we can. I've known writers who can send out a first draft and I'm aiming at that, not that I'm there.
That, I suspect, is the only way to really improve. Then look for the faults, get it as clean as you can and go on from there because picking over the same manuscript for years isn't going to improve your writing or get something new written.
So that's my theory on that.