Thesis
Be certain that your essay has a thesis.  In a short essay, it should appear at the end of the introductory paragraph.  A thesis is not merely a statement of the essay's general intent: This essay will explore the theme of nurture versus nature in Frankenstein.  It is rather a statement of the idea that the essay will prove: The story of the monster dramatizes the idea that an innocent nature can become evil through lack of proper nurture.

Topic Sentences
Be certain that the paragraphs within the body of your essay all have topic sentences.  A topic sentence is to a paragraph what a thesis is to an essay.  If you want to discern the logic of your essay's organization, try underlining your thesis statement and the topic sentences.  You should then see the coherent skeleton of the essay.  In other words, each topic sentence should relate to the thesis, and there should be an obvious reason why the topic sentences follow one another as they do.

Conclusions
The conclusion is the place in the paper to explore briefly the implications of  your argument.  Students sometimes have the notion that essays are supposed to begin broadly: In everyone's life . . .  Really, the reader wants the idea to come sharply into focus as quickly as possible.  If you must expatiate philosophically, do so in the conclusion.  Even so, the tighter such generalizations hug your essay's specific topic, the better!  Avoid large, moralistic conclusions about "real life" especially in analytical, interpretive essays about a work of literature.

Quotations
Be certain that you quote grammatically.  If, for instance, you incorporate a quotation within a sentence of your own, the resulting sentence should be grammatical.  A quotation separated grammatically from a sentence of your own should also compose a complete grammatical unit.  When using words not your own, place them within quotation marks.  Credit the source with a parenthetical citation.  To do otherwise is to plagiarize.

Major Errors
There are three "sentence level" errors that will keep your essays from earning better than a C in this course:  Frag--A fragment is simply not a complete sentence.  (The two most common unintentional fragments appear to be adverbial clauses and participial phrases.)  FS--A fused sentence really consists of two complete sentences that have been joined, fused, without benefit of a mark of punctuation or a coordinating conjunction.  CS--A comma splice occurs when two sentences are connected--are spliced together--by only a comma.  Often a semicolon is the necessary mark of punctuation.  As a colleague of mine helpfully insists, the semicolon is misnamed; it should be called the semiperiod.

Common Errors
Be alert to these errors: the dropping of apostrophes from possessive nouns, the confusing of it's (a contraction) with its (a possessive pronoun), and the mistaking of to lay (a transitive verb meaning "to put" or "to place") with to lie (an intransitive verb meaning "to recline").

Resources
http://www2.centenary.edu/resource.htm
There are at this address descriptive links to many useful sites.