A Statement of Purpose (SOP) required for undergraduate and postgraduate (MS, MA, MEng or MBA) study abroad, is probably the most difficult part of the entire application process. Tips articulated below will also help undergrad students to ace their SOP, however it is a little easier and more generalized. (An SOP may also be called an Application Essay, Objectives for Graduation Study, Personal Background, Cover Letter or some comparable title)
Remember your statement of purpose should portray you as:

(1) passionately interested in the field – by the ‘hook’ described earlier;

(2) intelligent – conveyed by overall writing, organization, expression;

(3) well-prepared academically and personally – can be conveyed by using the lingo of your chosen field (theoretical and practical knowledge);

(4) able to take on the challenges of postgrad school – can be shown by describing the rigorous grad school experience, challenges and relevant coursework and other accomplishments;

(5) able to have rapport with professors and fellow grad students – in other words, collegial – you can show yourself to be generally nice, helpful and cooperative – be true to your personality here;

(6) able to finish the graduate degree in a timely fashion – can be conveyed implicitly by your success thus far and more explicitly if you can tell some (brief) story about adverse obstacles you have overcome; and

(7) a potentially outstanding representative of that grad school in your future career – can be implied by your being an outstanding representative of your undergraduate school.