Originally Posted by subspacetom

NRT is the standard bearer for enforcing boarding order.

GAs hold up signs in advance and announce the 1k/GS/F, then C, then 1P/*G, etc order. Then they wade through the crowd and check boarding passes to make sure passengers are in the right area, and separate the boarding groups, politely but rigidly directing people accordingly. 1P/*G get the area at the front of the blue line, while 1k/GS/F are at the front of the red carpet line, with the C crowd behind.

Then they usually have at least two GAs doing the actual boarding once it starts so there isn't too bad a logjam at the scanner, while another keeps the order back in the crowd.