don't (do not!) use contractions (eg it's, he'll, it'd etc): always use the full form (it is/has, he will, it would/had).
don't use colloquial language or slang (eg kid, a lot of/lots of, cool)
always write as concisely as you can, with no irrelevant material or “waffle”.
generally avoid "phrasal verbs" (e.g. get off, get away with, put in etc): instead, use one word equivalents.
avoid common but vague words and phrases such as get, nice, thing. Your writing needs to be more precise.
avoid overuse of brackets; don’t use exclamation marks or dashes; avoid direct questions; don’t use “etc”.
always use capital letters appropriately and never use the type of language used in texting!