Don't repeat the same words more than necessary; use variations.
Don't use unnecessary words; consciously pare down each clause.
Try for some sonorous expressions. Let the word choice fit the thought (onomatopoeia).
Use allusions and comparisons--to other authors, books, events, frames of reference.
Don't distract the reader with troubling inaccuracies; check spellings of which you're less-than-certain and make sure you've correctly formed contractions and possessives.
Be sure phrases are ordered for minimum interruption of sentence flow consonant with the progression of meaning.
Phrases should be arranged to rise to significant points. As much as possible, tend to your qualifications and explanatory phrases first, so that the powerful words receive emphasis.
Be careful to check that modifying phrases are syntactically clear. Usage for "which," "that," and "who" must be exact. Demarcate appositional constructions, and distinguish restrictive from non-restrictive clauses.
*Don't lapse into comma spliced or run-on sentences; independent clauses must be connected by semi-colons or divided by a period.
Be sure every part of the sentence is grammatically parallel; if you are listing a series of items related in the same way to your main clauses, they must have parallel grammatical forms. Be sparing in your use of participial phrases, which almost by definition are wooden and awkward.
Successive sentences should not be similar. Try to condense several with meaningful connectives; the process helps you think about and define the relation between your ideas.
Use them. Two or three for each page is a rough guide.
They should be clearly demarcated at the beginning, by topical or transitional sentences. If possible, the last sentence in a paragraph should prepare for the next one.
Sentences should be arranged in ascending logical, narrative, or emotional sequence. Examples should follow a rising pattern of cumulative demonstration.
There should be a clear sequence to paragraphs. The essay should have a shape; it should demonstrate something, with examples from the text clearly explained (that is, you should note how each quotation is relevant to your point).
There should be grace and power to your language; readers must believe that your subject is one of importance both to you and to them; that you know whereof you speak; and that your evidence bears centrally on the issue(s) you have raised.
An essay may not seem to have all the individualizing features of a poem or a private confession but it is nonetheless an expression of its writer's personality. For reasons even more urgent than the desire to obtain higher course grades or other forms of external future advancement, for your own sake it matters greatly that you learn to find rhetorical forms that represent your unique self and convictions.