1) Once it's written, it's done. Revision is like taking a test in school: NEVER change your answer unless you have an absolute and incontrovertible argument as to why it should be changed, or if you have outside input as to why it should be changed. Don't second-guess yourself.
2) Write for yourself. It's nice to have readers. It's nice to have people pay money for your stories. But the stories are yours, parts of your time, your thoughts, your soul. Be ready to live with them.
3) There's plenty of terrible things in the real world. In the end, your invented world should be better than this one. Yes, bad things can -- and will -- happen. But at the end of the story, things should be better than they were.
4) BE CONSISTENT! You are creating a world. Worlds make sense if you understand their basic rules. Make your rules, then don't break them. If it LOOKS like you're breaking them, know WHY. Mysteries are for the reader, not for the writer. if you don't know the answer, you're not playing fair. (this can be bent for large worlds under construction -- but you should keep the "I don't know" borders as far as possible from the main "play area" in which you're writing.
5) Make it fun. Maybe some people like reading books that are gloomy and depressing, but I don't, and anyone reading what I like won't. Bad things happen to the heroes, but there's always got to be a break.
6) SF/F is about wonder. If you can't feel the sense of wonder, if you can't get a thrill down your spine or a sting of tears in your eye, you're doing something wrong.
7) There is no such thing as an original story. There's only some old story either done differently, or two or more stories stitched together like Frankenstein's monster. So don't beat on yourself for finding that Someone Else did the same thing; ask yourself if they did it the same way, or differently.
8) Paid work gets priority.
9) You can correct people about matters of fact; you can't correct them on opinions or feelings; they're right, you're wrong. Remember this when you read reviews.
10: If it's work, you're not doing it right.