Here are Mark Twain’s Rules of Writing (from Mark Twain’s scathing essay on the Literary Offenses of James Fenimore Cooper):

1. A tale shall accomplish something and arrive somewhere.

2. The episodes of a tale shall be necessary parts of the tale, and shall help develop it.

3. The personages in a tale shall be alive, except in the case of corpses, and that always the reader shall be able to tell the corpses from the others.

4. The personages in a tale, both dead and alive, shall exhibit a sufficient excuse for being there.

5. When the personages of a tale deal in conversation, the talk shall sound like human talk, and be talk such as human beings would be likely to talk in the given circumstances, and have a discoverable meaning, also a discoverable purpose, and a show of relevancy, and remain in the neighborhood of the subject in hand, and be interesting to the reader, and help out the tale, and stop when the people cannot think of anything more to say.

6. When the author describes the character of a personage in his tale, the conduct and conversation of that personage shall justify said description 

7. When a personage talks like an illustrated, gilt-edged, tree-calf, hand-tooled, seven-dollar Friendship’s Offering in the beginning of a paragraph, he shall not talk like a Negro minstrel at the end of it.

8. Crass stupidities shall not be played upon the reader by either the author or the people in the tale. 

9.The personages of a tale shall confine themselves to possibilities and let miracles alone; or, if they venture a miracle, the author must so plausibly set it forth as to make it look possible and reasonable.

10. The author shall make the reader feel a deep interest in the personages of his tale and their fate; and that he shall make the reader love the good people in the tale and hate the bad ones. 

11. The characters in tale shall be so clearly defined that the reader can tell beforehand what each will do in a given emergency.

12. An author should say what he is proposing to say, not merely come near it.

13. Use the right word, not its second cousin.

14. Eschew surplusage. 

15. Not omit necessary details.

16. Avoid slovenliness of form.

17. Use good grammar.

18.  Employ a simple, straightforward style.