Submitting short stories for publication is a good deal more difficult than most people would think... This is primarily because of the tremendous amount of record keeping involved that an author must do to ensure that he/she doesn't make a simultanious submission of a story.
Now, I've talked to enough writers to know that many of them consider the "NO SIMULTANEOUS SUBMISSIONS" warning that many publishers and editors include in their guidelines to be complete bullshit. From what I've heard, it's primarily there to keep writers from realizing how much market power their stories actually have.
When you think about it, it makes a certain amount of sense... the last thing that a magazine would want is to get into a bidding war over whatever brilliant piece of prose you've just written. It takes time, money, and negotiating skills. While as a writer this is the best case scenario, I can see how it would be the worst case for the editors.
So, I dutifully hew to the "sole submission" rule, and have over the years, filled spreadsheets and notebooks with dates of submission, contact information, and an endless array of editors comments and insights. It has in fact, effected what I produce to a huge extent, as I deliberately try not to make my pieces too timely or political... Some magazines literally take MONTHS to get back to you before you can send your prose to the next one, and by the time one has been accepted, it could be as much as two years out of date.
Plus that, I suppose that the argument could be made that all truly good speculative fiction should be timeless. I mean, have the works of Jules Verne become any less brilliant now that submarines and space travel are now commonplace? Will the current crop of aliens that writers have dreamed up be rendered foolish looking when we finally meet the real E.T.'s? These are the things I think about a lot when I write, and why my brand of sci-fi tends to so easily veer toward fantasy.
On the bright side, the rewards are worth the effort of patience. Even with my self-published stuff, knowing that others enjoy my stories is a great feeling. Achieving this on a national scale is an even better one. The most important thing is that you don't give up.