if you want to do high finance, MBB, certain functions of high tech, LDPs (assuming you aren't doing that before the MBA)
if want to switch function, country, industry, or a combo
then the easiest and at times the only way is getting an MBA
also, don't underestimate the power of just being around smart, driven, & like-minded individuals for 1-3 years.
that being said, getting an MBA is not always the answer. depending on your situation, getting certifications might be the better choice.
feel free to PM if you have questions regarding getting an MBA.