Read books and magazines about writing, go to writers' conferences, join a critique group. Read the type of books you want to write.
Know the basics.
Do you know what POV means and how to use it correctly?
Do you know how to use dialogue effectively and how to use actions and description as dialogue tags?
Do you know how to develop interesting characters?
Do you know how to balance action, dialogue and narrative?
Do you know how to get rid of your pet words and phrases?
If you plan on self-publishing at the very least your book should have:
Chapters that begin on the right hand page.
No page numbers on the first page of the chapter.
Alternating the title and the author's name at the top of the page, not appearing on the chapter page.
A separate page for each, title page, dedication and acknowledgements.
Indentation for the beginning of paragraphs, not 2 spaces between paragraphs.
(Save the 2 spaces to indicate a new scene or change of viewpoint.)
No matter how good the book is, if it's not fromatted right people do notice. It will cause a mark down in contests and by reviewers.
Get rid of repeated words and phrases.
Only keep what's important to the story.
Eliminate empty adverbs--use descriptive verbs instead.
Get rid of "to be" words, replace with active, vivid verbs.
Describe the scene as the POV character sees it.
Show what's going on--don't tell about if after the fact.
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Be careful what you say online.
Don't badmouth other publishers and authors.
And never get into a conflict with a reviewer.
Hope this helps someone.