Whenever I design a map, I just place letters (A,B,C) at notable locations. And they are merely a reference to a description. But that in itself doesn't tell my players anything. I also cover parts of the map, and gradually reveal areas as they explore the dungeon.
If you use GIMP, there is a fantastic little script over at Cartographers Guild that will turn any black and white (or any two color) map into an underground map automatically. Very cool stuff.