The most useful thing that I learned about writing; Number 1: I have learned not to get so bogged down in the "rules" of writing so that I forget to follow my instincts. I've learned no to adhere so much to the do's and don'ts of writing; have I used too much narrative and not enough dialogue? Does this story sound boring? Have I used too much dialogues and my characters sound like talking heads? I've learned to use what sounds right to me, to take the story to the next level to keep it moving.
Number 2: I've learned that I can not please everyone with my writing and that I should write something that pleases me and like minded people. If I dislike my own work, then most likely others will dislike my work. If anyone can get 100% of people to like their work, then they have struck gold, and they are destined for the best seller list.
Number 3: I find that if I write everyday, it's easier to tell whether what I have written is good or bad, and I can reread, edit and think.
Last but not least, if I read other author's work, painstakingly studying them for information, gathering their narratives and techniques, this can help me to kick start my creativity. When you have models to aspire to, you find that you are not alone in what you write.
I find, after reading other author's work, that we are more alike than we are different and that helps me.