From Halopedia, the Halo wiki
Corner riding is a way to get down very steep cliffs or walls in Halo: Combat Evolved, Halo 2 and Halo 3 campaign without dying. It can only be done down very steep surfaces by a right or acute angle, or a very small obtuse angle, if performed properly.
Stand near the corner of the roof on the building you want to get down from. Walk over the edge, and then use the small amount of control you still have while falling to move back and forth from one side of the corner to the other.
Every time you pass over the corner, you slow down a bit, resetting the fall timer. If done right, the timer can be reset endlessly, and you can fall as far as the wall goes.