Bunny Hop
It is possible to bunny hop your dirtbike. While not moving, hold L1 (clutch) + X (gas) + Analog-stick Down. Then, hold R2 or L2 (preload) until the bar is full. Then, release L1 (clutch) and if done correctly, you should take off into the air. Level yourself out and complete the bunnyhop. It may be a little tricky at first, but it is fun to jump on buses and and other things. You can also bunny hop, going slowly (or any time you can pull a wheelie). However, it is easiest when going very slow or staying still.
Tricks
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.