Go to the corner of the Apartments district by the music store. Almost right across the store is a big rock by the cliff. Grab a car (Police car is fine) then park it on top of the rock or on the left side so that the door to the driver's seat is facing the cliff.
Grab another car, and ram into the police car. Once you do this, get in the police car and you will be through the invisible barrier and you can walk off the map.
Note: If you get into the black area at the edge of the map, you will be unable to move and will have to end your game.