In my experience, I can't emphasize enough how important pro short fiction is, when breaking into the SF writing world. Why? Because it's possible. Even top markets will take unknown writers. It's not "Who do you know?" it's "How good is your writing?" So you have to write a great story. But what else?
For years I'd been told to read the market I was submitting to. Never did it, never sold anything. But when I did take this advice, I started selling. Why? Well, I had a clearer idea of the kind of story that market was looking for. Also, I think I unconsciously set my standards higher.
Rob Sawyer says, submit to top markets. His reasoning is, if you expect yourself to be at the top of the market, you'll live up to your own expectation. And . . . if a story doesn't sell, you still have secondary markets to try.
Scott Bakker gave me another tip: note when a publication gets a new editor--they want to discover their own new talent. Scott might've meant book publishers, but it worked for me in short fiction.
You've heard this before, but it's true: a rejected story might not be a rejected story if the editor gives you critique. I sold several stories that I re-submitted to the same editor after a rejection, once the editor's concerns were addressed.
Again, keep your stories in the mail! Keep a chart showing which market has rejected each of your stories, where that story is now, and which market you think it'll be suitable for next. They don't sell if they aren't out there.
Cover letter? What if you don't have any sales yet? My first cover letters listed the professionals I'd worked with. If you get a chance to sign up for a workshop at Renovation, for instance, put in your cover letter, "I worked with Author X." And remember: keep the letter short and professional.
Networking: At a worldcon party, a writer said to me: "write something short and funny for Analog." And you know? He was right. Both my Analog stories fit that description.
So, yeah. These are a few things that have worked for me, but I'm sure I'm only scratching the surface. I love forums like this because I'm always looking for new tips as well. So, give: what tips have worked for you?
2009 Prix Aurora Award finalist ("Back," Analog, June 2008), Susan Forest works as a fiction editor for Edge. Her stories have appeared in Analog, Asimov's, OnSpec, AE Science Fiction Review and Tesseracts. Catch her at www.speculative-fiction.ca.