Get some air, then hold the Trick button and press the indicated button(s) to do the corresponding move:
Bar Hop: Press Triangle(2).
Can Can: Press Square(2).
Cat Nac: Press X, Circle(2).
Catwalk: Press Square, Triangle.
Cliff Hanger: Press Circle, Triangle.
Coffin: Press Triangle, Circle.
Cordova: Press Circle, Triangle(2).
Disco Can: Press Square(3).
Heart Attack: Press X, Triangle.
Heart Breaker: Hold Analog-stick Down until you have done a complete flip.
Heel Clicker: Press Triangle.
Helicopter: Press X, Triangle(2).
Indian Air: Press X.
Kiss Of Death: Press Circle(2).
La-Z-Boy: Press Circle, X.
McMetz: Press Triangle(3).
Mulisha Air: Press X, Square.
Nac Nac: Press Square.
No Hander: Press Circle.
Nothing: Press Circle, Square .
Pendulum: Press Square, Circle.
Rocket Air: Press X, Circle.
Rodeo Air: Press Triangle, X.
Saran Wrap: Press Triangle, Square.
Seat Grab: Press X(2).
Suicide Flip: Hold Analog-stick Up until you have done a complete flip.
Superman Indian: Press X(3).
Switchblade: Press Square, X.
To unlock the option to choose between 125cc and 250cc, go through the game in career mode until the last level. Once there, acquire first place to unlock the option to change your cc's on your dirtbikes in any mode.
Start the game in two player mode. Have player one hold L2 + R1 and press Up, Down, Left, Right, Triangle. Then, have player two hold L2 + R2 + Triangle + X.