Professional labs use miniature can openers to avoid getting any fragments into the package since failure analysis may want to see if the device failed from particulates. For RE purposes a Dremel can suffice followed by a good clean.

http://www.youtube.com/watch?v=lt_zFq4qzd0

Sometimes dies are soldered to a substrate (eg: silver heatsink / carrier). Example after die was removed (by force):

Soak in nitric acid and it should begin to dissolve and come lose: