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. Don't never use a double negation. Do not put statements in the negative form.
Verbs have to agree with their subjects.
Proofread carefully to see if you words out. If you reread your work, you can find on rereading a great deal of repetition can be avoided by rereading and editing. A writer must not shift your point of view. And don't start a sentence with a conjunction. Place pronouns as close as possible, especially in long sentences, as of 10 or more words, to their antecedents.