One key to making them readable is organizing the various portions and positioning them in such a way so as to minimize the number of elbows, tees, and crossings in the connecting connecting wires (ie, lines). The topic at
viewtopic.php?f=1&t=2987 is somewhat related. It's ok to do a block diagram of the entire computer; but there's little use for putting everything in one schematic, just as you would not present a diagram of an automobile that showed an exploded view of a tail light assembly in the same diagram with an exploded view of the water pump or the windshield wiper motor. When showing such detail, separate smaller diagrams are more useful and appropriate.