No matter how good of a writer you are or how much effort you put into the essay, failing to establish a solid foundation of knowledge on the topic beforehand will hinder the overall quality. Before you even begin the writing preparation, take time to research and fully understand the assignment.
Waiting until the last minute to start on the essay is too risky to chance. Begin the assignment as early as possible to allow yourself the opportunity to make revisions. No matter how well the first draft comes out, there is always room for improvement. Starting early also ensures you will still hit your deadline if you have more trouble writing the essay than you anticipated.
Avoid heavy reliance on your thesaurus. Stick with words you would comfortably use in conversation. This lends some credibility to your essay and gives it a unique feel that you personally provided. Your points will come across much more effectively using familiar language, and the finished product will make for a much better read in the eyes of peers and teachers.
Do not hesitate to show your first draft to others, including friends, family and teachers. Extra sets of eyes will discover errors you never realized were there. You may also receive ideas or suggestions to improve the content of the essay that otherwise would never have occurred to you.
Without even meaning to, you may accidentally plagiarize one or more of your sources in the process of writing your essay. This is easy to do if you don't note your citations properly. In the course of researching and studying the topic, you may assimilate the ideas of others and put them in your own words a little too closely to the words of others. Be careful to give credit to your sources for everything except for the material you came up with on your own.