Pause during a game and press Right, Down, Right, Left, Right, Up, Right (x2). This will unlock everything in the game.
Daryl -- complete the game with all 20 wheels.
Frontflip: Press Up (x2), Square.
No-Footed Backflip: Press Down (x2), Square.
Rock and Roll: Press Up, Triangle at the lip of a ramp
Bluenose: Press Up, Right, Circle.
Backflip: Press Down (x2), Square.
Backside Boneless: Press Down, Triangle at the lip of a ramp.