Avoid unnecessary repetition in your writing.
Vary your vocabulary to keep the prose lively. Try not to use a particular word or phrase more than once in the space of a few sentences, unless the repetition adds rhetorical emphasis.
You also need to vary the syntax of your sentences. Use subordinate clauses, parallelism, and inverted syntax to break up the monotony of the standard subject-verb-object sentence structure. It is particularly important not to begin each sentence the same way: Weak: “Jonson argues that . . . . Jonson states that . . . . Jonson shows that . . . .” If the sentences are short, try combining them into a list: “Jonson shows A, B, and C.”
Do not cite any quotation more than once in your essay. Use a broad range of evidence to support your argument.
See also: Redundancy.