I know a lot of people in Belgium who tried to prepare for an MBA and Masters on their own. Most of them failed or managed to get into second-tier programs, which will not be enough for them to get the career that they envision. As a manager, you should identify your weaknesses and work with people who would be able to help you get into a good program. That is why I went to the MBA Center too. Don’t hesitate to work with professionals who train students for the GMAT every day and might be able to help you get a better score than the one you can get on your own.
Another drawback I had was that I was a bit late in my application, the reason being that in Belgium people finish their exams in late January of each year and go on vacation and I started my application in mid-February and I had very little time to prepare my applications and the GMAT. This is something that I would advise people not to do. Rather, start preparing in advance, probably during the summer of the year before so that you finish writing your application by October before starting school. Have a better schedule to maintain your timings.
I would also like to advise people to start your job search before your Masters because the program at LBS is very short and fast: you start in September and you finish by June of next year. I made the mistake of waiting too long to send my resume. I advise you to start sending out resumes as early as possible and also use the summer before the MBA to make contacts.
Finally, I would like to recommend MBA Center services to all prospective applicants. I had many friends who attended the GMAT classes at the MBA Center and managed to get accepted to top tier colleges like me. So it would be a worthwhile investment.