Just off the top of my head, without thinking about it too much:
1. Learn what the passive voice is and avoid it at all cost.
2. The simplest way to say something is usually the best way.
3. Learn to use a dictionary and a thesaurus, and be aware that you never use a thesaurus without also using a dictionary. I once had a student describe a rock as “suave,” and it took me a couple minutes to realize that he had chosen the word from a list of synonyms for “smooth.”
4. Spell-check does not catch words that are spelled correctly, but are not the right words. (viz. “looses” for “loses”