It is important to review your essay many times as you write it. If you have wrote the same word, like "great" in the same paragraph five times, then you should look for synonims of that. You can consult an online dictionary, like Thesaurus, that beyond giving you synonims to the term you are looking for, will provide you with examples of its usages.
Remember to always read your workpaper, as many times as necessary, because in each time your thoughts may be clearer and then your writing will improve.