Using a single basic theme is one method you can employ to guarantee your essay's coherence and effectiveness. It forces you to organize your thoughts and ideas and helps keep the essay focused. Without a theme, your essay will simply look like a haphazard collection of facts thrown together to impress the reader. With a main theme, the essay will have a focal point upon which you will develop the rest of the paragraphs.
After grammar and spelling, the next problem that needs be checked is transition. Read your essay. Go through it from end to end, and check to see if your essay flows smoothly. Does each sentence flow smoothly to the next? Does each paragraph flow logically to the next? Each statement should be connected or somehow related to the one before it, not thrown randomly together. Next, limit the use of "I" in the beginning of most sentences as it is already obvious that the essay is a piece written by an individual.