Time
It all happened pretty fast for me. I'd never even thought about being a writer until I chanced on an article about Robert A. Heinlein wherein he said he wrote his first short story to pay an overdue electric bill. I had plenty of overdue bills, so I figured what he could do I could at least try.
I was a high school dropout, and didn't know a comma from a coma, but I bought a grammar book and read it. Then I sat down and wrote a fairly long story in a couple of days and mailed the first draft (Who knew you were supposed to write more than one draft?) to my favorite magazine. A few weeks later they bought it, and paid almost as much as I earned in a month at my day job.
When the check arrived in the mail I took one look at it and quit my day job.
I wrote two more quick stories for a couple of other magazines, and they sold, as well. Then I sent a query letter and the first couple of pages of a novel I hadn't written yet to an agent, and she called almost immediately saying she wanted to see the complete novel. She said she needed it by the first of the next month, and instead of telling her it wasn't yet written, I said I needed to do one more quick draft. (Well, that was, as Huck Finn said of Mark Twain, mainly true. I did need to do one more quick draft. I just didn't tell her it was the first draft.)
I wrote the thing in three weeks because I had to, and mailed her the first draft. (I still didn't know writers were supposed to do more than one draft.) I mailed it to her on time, and she sold it not much over a month later.
It was all very fast, and I went from never even thinking about being a writer to being a pro actually earning a living at it in only a couple of months. I still find it all really strange.
I think sending stories to the top magazines first is the smartest thing any writer can possibly do. You do need to be serious about writing and selling, but why write and submit if you aren't? Starting at the top and working your way down is probably the oldest advice in writing, and the benefits greatly outweigh the disadvantages. If no one wants it, your story will hit the bottom of teh ladder sooner or later, but why rush it? Give someone a chance to buy it before it hits bottom.
You only get really good by competing with the very best, and by letting the top editors see what you've written.
And one thing is absolutely certain. . .if you start at the top and work your way down, your story will sell to the best magazine that wants it. If you start at the bottom and work your way up, your story will sell to the worst magazine that wants it.
There's no shame in being rejected by top magazines, and it's never a waste of time. The fastest way to break into any magazine is to try writing for that magazine, and to keep writing for it until the editor says yes. And the surest way to break into top magazines in general is to start at the top and work your way down.
Very few get good enough to write for the big boys by writing for the little ones. And if they do, it's a tediously slow process. If your stories are anywhere close to professional quality, some of the top editors will tell you where you're going wrong, and you can make changes accordingly.
Joyce Carol Oates was rejected thirty-five times by The Atlantic Monthly before C. Michael Curtis bought one of her stories, but once he did, she was suddenly hot.
Starting at the top and working your way down may take more postage and more patience, but if you don't buy stamps by the roll, and don't have a fifty gallon drum of patience in this business, you're going to shoot yourself in the foot. Or somewhere even more painful. Other than needing more postage and more patience, there are no other drawbacks to starting at the top and working your way down, and many, many advantages.