1. Be likable
Despite popular opinion, admissions officers do not necessarily get personal satisfaction from accepting a student with a 4.0 GPA and a 770 GMAT. What they enjoy is accepting students who, based on the interview and the essay, they like. If you can portray yourself as someone who the reader can cheer for/relate to/find interesting, then you have greatly helped your cause.
2. Don't try to over impress the reader.
Focus on sincerity and honest reflective assessments of your abilities. Try to avoid describing yourself as having a "unique ability"or "remarkable skills" unless it can be substantiated. Don't go to the other extreme and dwell on failures. Failures are "opportunities" and "challenges" from which you learn and develop. Don't use the essay space to discuss your GPA or GMAT.
3. Avoid overuse of "I"
"I would like to go to Kellogg Business School because I would like to study marketing and it would best suit my goals, so please accept me because I am remarkably talented."
The paradox of the admissions essays is that you have to write hundreds of words about yourself, yet you can't sound self-centered and narcissistic. Obviously you have to make references to yourself, but simply be conscious of not overdoing it.
A way of getting around this paradox is to SHOW the admissions officer what you have done instead of telling them how awesome and special you are. Let your actions speak for themselves.
4. Typos and tone
Typos are unacceptable and suggest a lack of professionalism. You should ideally have professional proofreaders review your statements for errors and MBA admissions consultants check for proper tone, content and style.
5. Trite phrases
Most admission officers are near nausea with applicants who "want to help people" or who "think outside of the box." Try to describe yourself in an original and interesting manner.
6. Jaded cynicism
Yes, we know the world is full of selfish jerks, but save your rant session for your blog. Colleges want bright and optimistic people. A positive approach to life will score points.
7. Am I done yet?
Respect the fact that essay readers must spend their days reading tiresome essays. Have mercy on them and keep your essay below the word limit. One source of waste is repeating content from different parts of the application.
8. The Dreaded B-School Lingo & Thesaurus Speak
The utilization of sophomoric language to delineate concept expression hinders, rather than facilitates comprehension, moreover, such methodologies generate antipathy and a soporific effect, as opposed to engendering any synergistic relationship between reader and essay.
Contrary to popular opinion, the overuse of polysyllabic words does not make up for a lousy GMAT Verbal score.
Writing in passive is another no-no. Try to put life and energy into your essays instead of making it dull, drab and pretentiously stilted.
9. Prepare early
"Could you help me rewrite this??? The essay has to be mailed out in 48 hours."
Be sure to start at least a few weeks (months?) before the essay is due. The admissions process does not favor procrastinators.
10. When in doubt, leave it out.
When you are "done" with your essays, you have yet to begin the toughest part. Here you have to slash out any unneeded phrases and paragraphs. Cut out any unnecessary material.
11. Know your audience.
Certain b-schools have certain specialties and cultures. A school that specializes in international business may not want to hear about how you developed your core values from growing up on a farm in a small town. Talking about your experience abroad and how it prepared you for your career in international business would be more appropriate.
12. Create a brand
If you can create a narrative and a brand across your essays around a cohesive theme this will it easier to make the admissions officers understand you. The conclusion of this theme will be the future. Describe to the admissions officers what you intend to do with your degree.