When you begin, go straight. There should be 2 ledges, go on the high ledge (they are covered with dirt). Keep going down the path holding no buttons but Up and Triangle. Keep holding the Triangle and Up buttons when you reach the little 1/4 pipe connecting to the wall. you should then be riding the wall, and if you fall, you will get a "player cratered" message. Cool!
Slim and the Bonus:
Beat the game with anyone and you will unlock a guy called Slim Jim and a bonus bike.
Unlock The Amish Boy:
Beat the game with all ten original riders.
Unlock Hidden Features:
Beat the game with riders other than D.Mirra, Slim Jim, or R. Nyquist to open modes such as night vision or exorcist mode.
Get Suspension Mode:
Beat the game with Chad Kagy.