Best advice I got (I haven't been picked up yet, however) was to write about two things:
1) Why you want to join the Navy (why the Navy will be good for you)
2) What you have to offer (why you will be good for the Navy)
Being specific and relating your life experiences to these two topics is probably what will distinguish a dry, idealistic essay from a personal, concise statement of purpose.
Of course, take my advice with a grain of salt, it's just a suggestion.