Echolocation, when I can't see the stars.
A serious answer this time : maps, real paper ones, usually torn from a spiral-bound road atlas.
Supplemented by an occasional look at Google Maps on a smartphone when I'm stumped, which happens mainly in towns when looking for the right road out.
On more local rides, especially if I'm leading a group, I just store a route in my head, using Google Maps at home to help prepare, and streetview for a preview of unfamiliar junctions so that I'll recognise them when I get there. I always have a folded map in my back pocket just in case though.