I will get to the answer in a moment, but let me answer an easier question first--what not to write in your essays.
Avoid obscenities, malapropisms, grammatical errors, and spelling errors.
Take the time to have many people read your application. Make sure they understand they are reading for grammar, and not content. Reviewing a paper for grammar requires a different set of techniques and uses a different part of the brain. To review for grammar it is actually best if sentences are read backwards. This prevents the brain from filling in holes. The brain is the first and greatest auto-correct. It will read words as they should be and not as they are. Reading backwards prevents the brain from engaging this process.
Do not use another school's name in your application.
Cutting and pasting from one school's application to another might make sense, but neglecting to read it again, and make the appropriate changes, is an error of sloppiness. At best it signals a lack of attention to detail, and at worst laziness.
I know this seems simple, but you might be surprised how often these rules are violated. I say rules because running afoul of these two rules is the quickest and most sure way to get your application denied.
Ok, now on to the question of content. Here I do not use the word rule, as there are many paths to one destination. This is just my suggestion as you start to think about what to write.
I have a good friend who coaches politicians on how to effectively handle media interviews. She tells them you can never know everything that may be asked, or queried, in an interview. So, you should draw a triangle and label it with three topics on which you are going to be very well versed and conversational. If a reporter asks you about a topic that is not one of your three, you gracefully find a way to redirect the question to be about one of the three topics on which you were prepared to answer. I am sure many of you have seen a political debate and observed when someone is not adept at this. You will shout at the television but they did not answer the question. Now you know, it was outside of their preparation triangle.
What does this have to do with your essays? Too many people use the essays to try and tell us their life history -- they cover birth to the present moment. While often a very interesting read, there is just not enough space in the essays to cover so much material and make us feel like we really got to know you in a deep and meaningful way.What we would rather have you do is to pick your three strengths, and use the essays as a vehicle to demonstrate those strengths.Are you the smartest person in the room? Are you the hardest worker? Do you possess outstanding leadership skills? Or are you the funniest person in the world, and great to have in a group? Whatever your three are, make sure the essays speak to your strengths by providing data and supporting stories.
When you are done, I suggest giving your application to two different sets of people (actually three, because remember, one group is only going to be reading for grammar). With the first group, tell them the three themes you are trying to disseminate in your application. Ask them to read it and then have them tell you if the three themes resonated clearly. For the second group, ask them to read your essays and tell you three themes that resonated with them. If the aided group gives affirms your strengths with their response, and the unaided group gives you words that are close to the strengths you wanted to disseminate, then you have a good application. If, however, the feedback is inconsistent, you need to go back and be more consistent, articulate, and streamlined.
Finally, we are often asked what about addressing holes, or areas of concern in my application? Should I say anything, or leave it unaddressed? You need to address areas of concern, holes in your application, or a bad grade on your transcript, for example. However, you do not, nor should you, use the entire application to make amends for a perceived, or real, weakness. You need to address an issue, explain what happened and why, but then move on. No school is going to admit you because you did a great job of explaining a weakness. They are going to admit you because of your strengths. Cover the weakness in some way, perhaps using the optional essay if available, and then move on to selling your three areas of strength.
By avoiding errors and aligning your application around three key strengths, you will give yourself the greatest chance of your application being memorable for all the right reasons.