Avoid alliteration. Always.
Prepositions are not words to end sentences with.
Avoid cliches like the plague. (They're old hat.)
Employ the vernacular.
Eschew ampersands & abbreviations, etc.
Parenthetical remarks (however relevant) are unnecessary.
It is wrong to ever split an infinitive.
Contractions aren't necessary.
Foreign words and phrases are not apropos.
One should never generalize.
Eliminate quotations. As Ralph Waldo Emerson said, "I hate quotations. Tell me what you know."
Comparisons are as bad as cliches.
Don't be redundant; don't use more words than necessary; it's highly superfluous.
Be more or less specific.
Understatement is always best.
One-word sentences? Eliminate.
Analogies in writing are like feathers on a snake.
The passive voice is to be avoided.
Go around the barn at high noon to avoid colloquialisms.
Even if a mixed metaphor sings, it should be derailed.
Who needs rhetorical questions?
Exaggeration is a billion times worse than understatement.