When you want to walldrive you should go at least 100 mph, then drive next to a straight wall (i.e. drive on the pavement next to a building) and steer your car into the wall, not too fast but also not too slow. This should send you driving up against the wall.
You won't drive for a long time, only one or two seconds, but then you will start flying through the air and this is where the real fun begins. You can fly over the freeway wall or land on top of a building.
But the programmers of the game did not expect you to do that, so most of the times you just crash through the building, the screen will look very weird, and you will be dropped on the road again.
To walldrive we prefer to use the Subaru Impreza or the Mitsubishi and we set the physics realism all the way to the left.
To learn a new trick it's best to drive in cruise mode with no other traffic, and also to use the nodamage-trick (press CTRL-ALT-SHIFT-F7 and type /nodamage).