There is often a specific question asked on MBA applications, so the first thing is not dodging the question or dropping a standard essay that you use (or slightly adapt) for all of your applications.

But I think you are right to start gathering your thoughts.

Most questions I've seen are some variant of: "Why an MBA? Why now?" So, thinking about that is where I am starting. And even if you don't use it on the written essays, you will almost certainly be asked in an interview (so, it is not wasted mental energy)

The other standard-ish question would touch on some formative experience in your career, and perhaps how an MBA could help (or have helped) you cope better with certain challenges.

I would suggest you find some books, which talk about MBA application essays. One such book is "The great application essays for business schools" by Paul Bodine-

http://www.amazon.com/Great-Application-Essays-Business-School/dp/0071452990

This will provide you exposure to various MBA app questions, and their relevance. I have done a fair amount of research and below are my findings -

1. Career goal and related essays (why MBA, why now, why this school etc) are the most important essays in an application. Career goal should make sense to the reader. In most of the cases this is the first essay admission committee reviews. If you falter here your chance of getting accepted becomes very slim.
2. Whoever you talk to will ask you to distinguish yourself from rest of the applicants. I have often wondered about it. Schools get hundreds of applications and its not possible for anyone to know profile of other candidates. So how do you distinguish yourself? The best way of doing that is to show the human side of you. Some career related stuffs can also be helpful but most of the times it?s your personal stories that impress admission committees. Did you overcome an obstacle, which could have spoiled your dream? If yes then how you overcame it and most importantly what did you learn from that experience? It would be even more impressive if you could show how your experience will enrich experiences of your future classmates.
3. Make sure that your application as a whole makes sense and is able to provide clear picture of you, your goals and accomplishments.
4. Always remember that schools want something from you as well.They want you to contribute to school. try to show how you can.

Writing MBA essays is a time consuming and stressful exercise. But once you have honestly written them you will learn a number of things about you, which you would have never thought important. The process does help you improve your awareness and overall perspective.

Few words of caution ?

1.Don?t chase rankings of a school. Ranking could be one of the criterions but cannot be the sole one. Once you have identified colleges try to learn more about the program by talking to current students or alumni and attending school events. And then make the decision.
2.When you look at the school look at the essay questions, and see if those will provide you enough space to convey about you and your personality effectively. If not then look for other school unless you are very sure that you have things that schools are looking for anyways. One example is Darden Business School. This year they had an essay on leadership and two very short essays on career goal and your contribution. In my view such applications don?t give enough space to distinguish you. I am very sure Darden is going to come up with better set of questions next time.

Overall it is very difficult to find out what colleges are looking for. They have various parameters, and sometimes depending on your background you will have to sweat more. For example if you are an Indian IT male then you are going to have a very tough time getting into a B school of your choice. The reason is ? they generally have very high GMAT score, and a similar profile.

But don?t lose heart and keep trying until you have made into one. My friend who attended Kellogg Management School put it very nicely for me ?

?B Schools are like women. If one does not like you move on to next?. And you will never know why they did not like you. Even if they tell you, you will be more surprised than satisfied.

All the best!

You've been given some great advice, and I'd just like to add a couple of things:

While it's ok to craft a relatively generic letter, you really need to include specifics about the schools you apply for. As you research schools, figure out what individual characteristics about the programs attract you and include them in the essays. Perhaps the worst thing you could do is send purely generic essays, because adcomms want to see why you think you fit with their school specifically, as opposed to any MBA program.

Secondly, if English is not your first language, have a native speaker proofread your essay if possible. You might have a great essay, but small errors in language or grammar can make big impressions on the people who read it.