dont want to mislead you. writing essay is truly time consuming, at least for me. so to prepare as early as possible. i think even if you are working on your gmat and toefl, you can still take some time to think for a while your past stories between your study. then when you really begin to write, you will have a rough idea and the process may be facilitated.
I am thinking of some stories when taking subway or lunch time. It's easier to have inspiration than make up a whole and logical story. for example on career development plan. thanks idaisy.