Never use a metaphor, simile, or other figure of speech which you are used to seeing in print.
Never use a long word where a short one will do. If it is possible to cut a word out, always cut it out.
Never use the passive voice where you can use the active.
Never use a foreign phrase, a scientific word, or a jargon word if you can think of an everyday English equivalent.
Break any of these rules sooner than say anything outright barbarous.