What best contributes to clear writing?

Active sentence construction. You never want o repeat vocabulary or subjects, or your writing will become boring. Always use complete sentences including a strong subject and predicate. State your point with logic and good explaination. If it is an essay, be sure to state all reasons with large enough paragraphs to get the point across. If your sentences are too complex it could result in straying away from the actual point or even run/ on sentences which are never good in important essays or business letters. Impressive complex vocabulary is good. Also, yes, it does streghten the content of the writing but too much or too little could potentially confuse your readers or even yourself if you use the words incorrectly.