It's snowmageddon here in Western New York, so I've spent a chunk of the morning doing some drawing, and roughed out this map. I wanted to get away from square/rectangular rooms and to create a 'great hall'; I started with the central corridor and built off of it. I think that this was the abode of a powerful sorcerer and his followers. He summoned something in the chamber to the east, drove it southward and collapsed the tunnels upon it to destroy it (hence the rubble in the SE corner of the map).
My idea of a megadungeon is always flavored by my experiences in Darkness Falls when playing Dark Age of Camelot (almost a decade ago, now... wow how time flies). I liked it for the big, broad caverns - and the sense that you were always delving ever deeper and deeper into darkness.
This idea of developing the megadungeon as 6-12 area sub-dungeons that all interconnect helps me to visualize the thing. I don't have one giant map to rough out and put together; instead, I'm left with a series of small areas (each with a unique theme, hopefully) that link together to former the larger superstructure.
Now, if only I could convince myself to put in some doors at some point (other than the odd secret door or two)...