Modifying a head to allow for removal of the face after assembly is an important step in building Gunpla, as it allows for painting of the face and sensor outside the head.
In this tutorial, I demonstrate how to modify the head of the MG Shin Musha to allow for the face to be installed later. This method isn't 100% for every kit, but the fundamentals can be implemented on other kits to achieve similar results.
Link:
http://www.ghostofzeon.com/diy/modifications/headmod.html(There is also a small video embedded on the bottom of the page - it is the first video I have done for a tutorial and hopefully not the last!)