I've published a collection of short stories that has been favorably reviewed and am hopeful of accruing mainstream publishing cred soon. Thus, what follows isn't anything so grandiose as "advice" so much as an essay on my own process and the things I currently believe about being a writer.
First and foremost, I don't plan on getting rich off my writing. Having been a gigging classical musician for more than a decade, I can honestly say that it seems the things most worth doing -- the things most valuable to our society and culture -- don't pay well. In the U.S., at least, there is a pervasive attitude that if we love what we do we should be willing to work for almost nothing. As far as I can tell, unless you're one of the lucky ones, there just isn't money in writing.
Second, write about what you know. It's so cliché and it's nauseating to even repeat it, but still it's something that needs repeating over and over and over. If you're a midwestern middle-aged white man, don't write about a teenaged African girl sold into sex slavery unless you know a whole hell of a lot about such a girl's experience. Does this mean you have to write autobiographical pieces? No. However, I feel as if I'm cheating my readers by straying too far afield.
My writing is about my voice and dealing with issues or expressing ideas I feel are important. The more real you can make your characters and world, the better the writing and the more audiences will connect. In many cases that requires first-hand experience or a lot of research. If you're from a small town in Oklahoma, you don't need to necessarily set your story there, but that regional culture may be very different from a small town in Massachusetts, so set your story somewhere similar.
Third, being in a graduate program requiring a lot of historical research, I've come to see a lot of parallels between writing research papers and writing fiction. For one, you have to do a lot more research than will go into the final document. In fiction, that means I have to be an expert on my setting and my characters. I can't half-ass it. While I can model characters after those I find appealing in television, movies, or other fiction, my characters have to be their own unique individuals and I as their creator need to know more about them than may ever go onto a page. If I don't know enough about them to make them three-dimensional, a reader certainly won't.
I work in fantasy genres, so I have to know the world, its rules, its inhabitants, and everything else about it. As a long-time RPG fan I use a game system I've developed over time that borrows elements from many different sources. I then use spreadsheets and character dossiers to keep track of information. The world doesn't necessarily need to be complex, but the further away from our real-world experience you go the less you can leave for the reader to assume. This is a lot of backstory work, lots of reading, internet searching, google earthing -- which is one of my favorite things -- and lots and lots of imagining. This is part of the hard work that goes into writing fiction.
Fourth, I had a conversation recently with a friend who is an academic, but who writes short stories in his spare time. He said he struggles to define the voice of his characters, that he feels like the all sound the same. I'm not sure I've ever struggled with character voice, but I imagine that one of the things that attracts people to LARPing and gaming is probably the ability to adopt different characters, play dress-up, and to be someone else. What I said to my friend was that almost all my characters -- or at least central characters -- are based on some aspect of me. It may be a small aspect of my personality or some small part of my life-experience. Despite this, they have to be individuals, so I try to imagine what I would do or say if I had their backstory and was in their situation. This would affect how they feel, how they dressed, their speech pattern, what details about a person or place stand out to them, or maybe even the way they structured their thought process.
Fifth, the editing and revision process can be torture. It may be the most brutal aspect of writing a story -- at least, it is before you have to deal with the business of selling it. One of the "Aha!" moments I've had was when a writer on a panel at a convention said your first draft isn't when you start writing or even when you've got it all laid out in front of you. Until you are ready to show someone else your work, it's all draft zero. You can go through as many draft zeros as you need to before you feel it's ready to show anyone else. No one knows or cares how many revisions you go through or how much work you put into it, they only care about the final product. Since having to do a lot of revisions and edits of my school papers I've come to have a newfound respect for editing. I've also come to enjoy the ability to tweak characters and dialogue, to elaborate on descriptions and give my characters and setting more depth.
And lastly, I've been working with a developmental editor for a while. She helped me with my short story anthology and has helped me with my first novel manuscript. There are all kinds of editors out there, none of them cheap and not all of them very good. My editor helps fix plot-holes, flesh out characters, and polish my stories as well as fixing grammar, spelling, and syntax. Anyone serious about writing should realize they will never be able to do it all on their own and it may be necessary to have someone with specialized skills in that area to edit your work. If you get a contract with a big publisher they have editors. If you're self-published or unpublished, the investment in an independent editor is more than worth it. Again, in the long-run the final product is what matters. If you are serious about getting published than you should do whatever you have to in order to give yourself the best shot possible. The idea that you should be able to do it on your own if you actually have talent or are "good" is obstructive thinking. As I've gotten older I've learned how toxic the word "should" is and try to avoid it at all costs.