Most essays are dramatically improved by careful editing. Good essays are the product of writing and rewriting. If you have time, put your essay aside for a few days before you begin to edit. This gives you time to gain a perspective on what you have written and to think further about your answer and arguments.
For more about editing, see our Editing Checklist.
Don't despair when you find faults in your essay - this is part of the editing process. If you find that you need more information, or your argument has holes in it, keep calm and concentrate on fixing any problems.
Once you have a well-organised and fairly complete draft:
Links
Proofreading Tips University of Arkansas Online Writing Lab
Proofreading Strategies Purdue University OWL
How to Proofread and Edit Your Writing: A Guide for Student WritersBemidji State University Writing Resource Center
Common Errors in English From Paul Brians, Professor of English, Washington State University