[Moejoe]

Heinlein, as with a lot of those authors, was under a form of patronage that you don't see occur any longer. The editors at the short story magazines and the publishing imprints were a much more selective and pro-active bunch back when Heinlein began his career. If they liked an authors work they would actively push that work, no matter the generated sales. That doesn't happen now, or at least, not quite as often.

I remember talking to an agent, and this wasn't that long ago, at a bar in London (can't remember how that all came about). We were discussing what it takes to get representation, and she said without any irony:

"There's only three ways to break in. You either know someone, you're known, or you copy something that's already well known."

It explained a lot to me about what book publishing is about, and why it's so cutthroat. They're not selling hearty meals here, they're selling Pot Noodles. Some celebrity jackass will get a million pound advance on a ghost written book about what they eat for breakfast that morning. If you're the nephew of the cousin of the publisher, you've also got a good chance too. And if all else fails, just buy a James Patterson novel and copy that.

EDIT: and on the subject of having representation. I seem to recall that there are 8,000 publishers in the continental United States, only 12 of them insist on you having an agent. (My memory might be fuzzy, but the figures are in the ballpark).