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.
Another important role of the PS is that it allows you to explain, although briefly, some information that may not be accurately represented by your transcript or Curriculum Vitae. For example, your GPA might be low because your school places a cap on grades given to students belonging to your major. Another example would be explaining the connection between your undergraduate major and your chosen graduate major, which might seem incongruous at first inspection. For these reasons alone, it is extremely important that you follow a guideline in composing your own PS.