Use words correctly, irregardless of how others use them. Understatement is always the absolute best way to put forth earth shaking ideas. Puns are for children, not groan readers. Go around the barn at high noon to avoid colloquialisms. Even IF a mixed metaphor sings, it should be derailed.
Exaggeration is a billion times worse than understatement.
The passive voice should never be used. Do not put statements in the negative form. Verbs have to agree with their subjects. A writer must not shift your point of view. Place pronouns as close as possible, especially in long sentences of 10 or more words, to their antecedents.