My favourite of these rules was one which my old RE teacher once wrote on one of my essays. I never worked out whether she had done it on purpose or not. She wrote, "You should never use the words 'always' and 'never', because there's always an exception". Here are some more:
Verbs HAS to agree with their subjects.
Don't use a preposition to end a sentence with.
And don't start a sentence with a conjunction.
It is wrong to ever split an infinitive.
Avoid cliches like the plague. (They're old hat)
Parenthetical remarks (however relevant) are (usually) unnecessary.
Also too, never, ever use repetitive redundancies.
No sentence fragments.
Contractions aren't necessary and shouldn't be used.
Comparisons are as bad as cliches.
Don't use no double negatives.
Eschew ampersands & abbreviations, etc.
One-word sentences? Eliminate.
Analogies in writing are like feathers on a snake.
The active voice is preferred. The passive voice is to be avoided.
Never use a big word when a diminutive one would suffice.
Kill all exclamation points!!!
Use words correctly, irregardless of how others use them.
Eliminate quotations. As Ralph Waldo Emerson said, "I hate quotations. Tell me what you know."
If you've heard it once, you've heard it a thousand times: Resist hyperbole; not one writer in a million can use it correctly.
Even IF a mixed metaphor sings, it should be derailed.
Who needs rhetorical questions?
Exaggeration is a billion times worse than understatement.
A writer mustn't shift your point of view.
Always check to see if you have any words out.
When dangling, don't use participles.
And I can't leave you without mentioning my favourite sentence-ending-with-prepositions. A father was putting his small boy to bed, and the boy realised he had left his book of bedtime stories downstairs. The father went downstairs to get it, but came back with the wrong book, and the boy said, "Oh no, what did you bring that book I don't want you to read out of up for?"