Avoid alliteration. Always.
Prepositions are not words to end sentences with.
Avoid clichés like the plague. (They're old hat.)
Comparisons are as bad as clichés.
Be more or less specific.
One should never generalize.
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.
Eliminate quotations. As Ralph Waldo Emerson said, "I hate quotations. Tell me what you know."
Employ the vernacular.
Don't be redundant; don't use more words than necessary; it's highly superfluous.
One-word sentences? Eliminate.
Understatement is always best.
Eschew ampersands & abbreviations, etc.
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?