"Around about then, both Isaac and I formed the habit of visiting science-fiction editors in their offices. Isaac concentrated on a single one, John Campbell, who had recently replaced F. Orlin Tremaine as editor of Astounding.
What Isaac did was write an actual story, leave it with Campbell and come back a month later to get the rejected manuscript (which he then mailed off to Amazing Stories, who bought it right away), along with a thirty-minute lecture on what Isaac did wrong and what he should have done right. So Isaac wrote a second story, trying to do it as Campbell had described. That got the same treatment; bounce with lecture from Campbell, acceptance by Amazing. And the third story was the charm. It was accepted by Campbell, as were scores of others over the next decades."
He also talks about how during WWII Asimov teamed up with Robert A. Heinlein and L. Sprague de Camp to work at a Naval research facility. One of the comments nails it. "Hmmm . . . famous SF authors working on weird technologies . . . that would make a great SF series."