First and foremost, it is important for the applicant to be clear about career goals and what his/her expectations from the MBA are. Then it's easy to shortlist and narrow down on a set of schools that are right for you. That said, there are a few generic guidelines that one can use to evaluate an MBA program and its suitability for oneself.
1. Rankings - Use them. But don't place too much importance on them. Every year, enthusiastic people around the world put substantial amount of thought and research into calculating those numbers, and it can be a mistake not to make use of their hard work. A broad variety of factors - quality of students, faculty, placements (career opportunities post-MBA), course design, infrastructure - are considered when a top-10 / top-100 list is prepared. While that doesn't mean that a school ranked 8th is inferior to the one ranked 7th, the ranked list does give you a starting point to conduct your own research and evaluate each program one by one.
2. Batch profile - Have a peek at your future classroom. If there's a significant 'disconnect' between your background and that of the admitted class, you may want to reconsider applying unless of course, you wish to switch your career track and the profile of the admitted class is actually the one that's of interest to you.
3. Fees - Perhaps the most important criteria for an applicant! But do consider the scholarship / aid opportunities that a school offers, and don't let the high fees scare you away from a scholarship-rich school. Look at the percentage of students getting financial aid.
4. Ties among the alumni - Networking is an important aspect and benefit of an MBA, and you should check whether the alumni actually remain in touch. Check whether there's a website / portal within the school's website for the alumni. May be even a Yahoo group? Check how active that website / forum / group is. How many 'meets' / events were held during the past year? Try to get answers to all these questions.
5. Location - In today's 'global village', this factor would probably be the last one on one's list, but it does make a difference albeit by a small bit. For example if the world of Finance allures you, schools in Mumbai (if studying in India), New York, London would get a preference, all other factors being equal.