Hi Somak,
I'll take the first plunge. I graduated in B.S. in EE and CS(and almost another major in Bus/IT), started my own company, then joined a big online retailer. There, I decided I want to go back into academia and business, but in what, I didn't know. I knew I liked a few things - math(and hence econ), technology, and an interest in marketing.
I was fortunate enough to find a (albeit low-paying) research position in marketing at a top tier school for a summer. That opportunity lead to good connections, contacts, and advice from my advisor there. He pointed out the possible paths I could take to study, and also what I wanted to do for a career path - econs, IT, and marketing. All three had topics which are relevant to my interest, and it was up to me to determine which I wanted to do. I researched about those fields, and decided to go for marketing.
In my SOP and talking with my recommenders, I try to convey why I chose marketing, and even a brief synposis on the process I took to come to that conclusion. I truly believe that the combination of research experience, and knowing WHY I choose marketing over other options(including not going to a PhD program at all) that helped me gain admissions to a top tier program. My scores are not stellar, but one of my recommenders from the research project helped tremendously.
I guess the only other advice is to try to put yourself in the admissions shoes. They are experienced application readers who are well informed about the subject, and know what to look for. They have 60-100 applicants each year(compounded by X years) for 1-2 positions. Some go directly to the recycle bin(bad scores, bad gpa, bottom tier school, etc...), some need to take a closer look, and about a dozen or so are strong candidates. The question then becomes, why marketing(or any specific field, he/she know what it's about?), why school X, does this candidate truly know what he/she is getting into?(high drop out rates), and is the candidate the best fit for the department/school/research? If you were taking applications, what would you look for? Perhaps that's the first true test of marketing - marketing yourself!
good luck!