Remember that when you change ideas, you have to change paragraphs in the essay. Keep your ideas clear, and you can do it best with short sentences.
Vary your sentence length. Try the formula that for every three short sentences, you can write one medium length sentence, and for every two medium length ones, you are allowed one long one. Avoid run-on sentences and fragments at all costs.
Pay attention to word order. If you are not sure if a word is a subject, a verb or an object, fill in the blanks for the question: "Who is doing what to whom?" The one doing the action is the subject and the object is having the action done to her.
Check your work for common grammar mistakes, including subject and verb agreement, using "that" before a restrictive clause and "which" before a nonrestrictive clause.
Remember the only time you use an apostrophe is for a contraction or to show possession. The grammar rule is that "it's" -- with an apostrophe -- means "it is" and "its" -- without an apostrophe -- means "belongs to."
Read your work aloud. Remember that commas are used to define a certain sentence. "The monks who were running, jumped aside" (nonrestrictive because every monk is running) as opposed to "The monks who were running jumped aside (restrictive because only a certain number of monks were running).