Writer's Rules, OK?

George Orwell
George Orwell reckoned that writers "need rules to rely on when instinct fails".
He proposed the following rules. (Slightly modernized here.)
1.    Never use a long word where a short one will do.
2.    If you can cut a word out, always cut it out.
3.    Never use the passive where you can use the active.
4.    Never use a foreign phrase or jargon if there is an everyday English equivalent.
5.    Never use a metaphor that has become a cliché from overuse.
6.    Break any rule rather than say anything outright barbarous.

Mark Twain
Mark Twain wrote (in a letter):
"I notice you use plain simple, language, short words, and brief sentences. That is the way to write English."

Rules are always controversial. But all writers need these things to cling to.
The following rules make a good litmus test. In general, if you can understand how these sentences break the very rules they promote, most of the material on this site will be understandable.
The point of many of these sentences is very hard to understand for those who use English as their second language, but mostly obvious to educated natives.

These rules are normally attributed to William Safire, "the most widely read writer on the English language".

01. Remember to never split an infinitive.

02. The passive voice should never be used.

03. Do not put statements in the negative form.

04. Verbs have to agree with their subjects.

05. Proofread carefully to see if you words out.

06. If you reread your work, you can find on rereading a great deal of repetition can be avoided by rereading and editing.

07. A writer must not shift your point of view.

08. And don't start a sentence with a conjunction.

09. Remember, too, a preposition is a terrible word to end a sentence with.)

10. Don't overuse exclamation marks !!

11. Place pronouns as close as possible, especially in long sentences, as of 10 or more words, to their antecedents.

12. Writing carefully, dangling participles must be avoided.

13. If any word is improper at the end of a sentence, a linking verb is.

14. Take the bull by the hand and avoid mixing metaphors.

15. Avoid trendy locutions that sound flaky.

16. Everyone should be careful to use a singular pronoun with singular nouns in their writing.

17. Always pick on the correct idiom.

18. The adverb always follows the verb.

19. Last but not least, avoid clichés like the plague; seek viable alternatives. 

Unfortunately, life and writing are seldom simple, and despite its obvious brilliance, (7,15,17 in particular) Safire's list has attracted some criticism. 4 is particularly controversial, 5 & 6 might be combined, 9 could be "improved" by putting the period outside the bracket, etc.

Conservation
This story is sometimes attributed to George Bernard Shaw.  
(This concept is explained in book three of "Editing and Design. A Five-Volume Manual of English, Typography and Layout", by Harold Evans.)

A fishmonger had a sign that said: FRESH FISH SOLD HERE
The fishmonger had a friend who persuaded him to rub out [erase] the word FRESH because naturally he wouldn't expect to sell fish that wasn't fresh; to rub out the word HERE because naturally he's selling it here, in the shop; to rub out the word SOLD because naturally he isn't giving it away; and finally to rub out the word FISH because you can smell it a mile off.
http://onemansblog.com/2007/11/02/improve-your-writing-with-conservation...

Bernard Shaw (GBS) and Eric Blair (O) imaginatively discourse on the phrase:
"A kind of Wikipedia® for Freeware"

(GBS)     "It's redundant"
(O)    "Of course, telling the truth in a time of war is a revolutionary act!"
(GBS)     "For Freeware, can go, straight away. It just repeats what's said in both the line above and below"
(O)    "And the trademark glyph. Who cares?"
(GBS)     "So it becomes ...  A kind of Wikipedia"
(O)    "It's not really. More like a wiki"
(GBS)    "A kind of wiki .... hmmm, but all wikis are "kind of wikis""
(O)    "So, if we remove the indefinite article, it becomes "wiki"
(GBS)    "Scarcely worth saying."
(O)     "Whoosh-cluck & it's gone."