Avoid Cliches. There are a few standard reasons why people become doctors, and so it is very easy to fall into cliched expressions and ideas. Though it's fine to express a commonly_held notion, try to find a personal way to express it. For example, rather than write "I want to be a doctor because I want to help people," discuss the ways that you have shown your generosity and kindheartedness in the past; bring specific examples__such as your volunteer experiences__so as to avoid a direct (and possibly cliched) declaration of your point. Sometimes cliches are unavoidable; just make sure your essay isn't full of them.
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.