Personally, the whole process for me was rushed!!!
Initial research, study for GMAT, sitting exams through to completion of all applications came hand in hand with the global release of the product i was working on and the whole process took 2 months for me. Not enough time. (At my age, applying soon was important, so it had to be done quickly)
So I would say, preperation is key! Most people here probably had around 6 months or more to get things done.
1) Make sure you know why you want or need an MBA and give a quick research into schools
2) Study and practise more for GMAT.. (This was my main time compressed area, therefore i did not do as well as i could have)
3) Create your list of schools .. Fit Fit Fit... No point applying to a Elite school if the curriculum, students and teaching style means you won't get the most out of it. After the MBA is over, the skills you have acquired coupled with your previous experience is what will push you on, so regardless of the institution, if you haven't maximised your potential due to bad fit, you won't get where you want to.
4) It's good to have some people who are good at English and NOT involved in your industry or profession to review your essays after you have written them, They can tell you which areas make no sense, where grammar has gone badly wrong and spelling mistakes. If it's understandable to them, then you know you haven't thrown in some technical jargon which an adcom may not understand.
In my case, my wife is a teacher, so has no technical background whatsover, so any 'tech' talk in my essays had to be written in a way that she could understand.
5) Thesaurus... try not to overuse the same words.
6) Take your time over the essays. Mine were all rushed. I banged out the first 12 essays within 1 week to meet my first deadline!!! I only had time to make 1 draft for all the schools I applied to. Definitely give yourself time to plan the essay, but don't agonise over it. I certainly believe that some essays, if re-written too many times, can appear over-engineered. A few style 'flaws' are not a bad thing imo, it may even reveal more about your personality. That's why there are writing analysts in certain institutions.
7) After you submit, don't worry. There not much else you can do. Panicking won't help you any.
For me, I just wished i realised i needed an MBA earlier than mid-October