It's just human nature that people like stories. Anyone can string together a pile of platitudes and adjectives to tell you outright what they think about themselves. But how boring, common, and uninspiring that would be! You want to leave your reader with a better understanding of who you are as a person and how you see and interact with the world.
All great managers know that behavior matter more than traits. Therefore, it is useless to try and describe yourself with words like "hard working," "passionate," "great personality," and so on. Meanwhile, the best writers know that great writing does not involve telling someone the point of the writing, but presenting the story in a compelling way so that the reader arrives at the message themselves. Image how boring poetry or satire would be if they just told you what the point of the work is. Therefore, focus on creativity and self expression. If you have a creative idea (and you should if you've followed this guide and lived an interesting life up to this point) and write your essay well, this positive opinion about you will take care of itself.
Example - One student who rode his bike from San Diego to Stanford and wrote his essay on his experiences during the bike trip. Another person wrote page 83 of his future biography. My friend wrote about how the tv show Seinfeld changed his life and reflected certain aspects of his personality. As you can see, there are a number of creative ways to go about writing your essay, so start thinking early and decide on something that admissions officers will remember.
* Story of a life changing experience - You should have had at least one instance in your life that you know changed you fundamentally. If you've followed this guide and actively sought new experiences, better yourself, and challenge your comfort zones, this should come easy. Make it powerful, include dialogue, and mention specific things about you that this experience affected.
* A meaningful conversation - Writing your essay in the form of a conversation and use it to tell an important story about you.
* Choose a strange pretense - One surefire way to capture a reader's interest is to write about something relevant to you in a wacky format. For example, you could write your essay in the format of a random page of your eventual biography and start your essay with only part of a word that is implied to be continued from the previous page. Just be careful if you decide to go this route; a unique format is nice, but execution is far more important. Don't think that a good gimmick will make up for lack of substance or poor writing.
Of course, all of this assumes you choose the open ended topic. If you end up writing to a predetermined topic since schools that don't take the Common App have their own questions, simply use the same general rules. Don't write about things that you did in an expository way. Instead, try to tell a story and make entertaining, memorable, and honest.