Unlock all bonuses:
When you are at the main menu, hold L1 + L2 + R1 + R2 + R3 (Depressed + Up)
+ L3 (Depressed + Down). If the code is accepted you will hear a sound.
Note: If you enable this code you cannot progress any further in the game. 
If you win a race it will not count.

Big Head mode:
Press Start to pause game play, then hold L1 + L2 + R1 + R2 + R3 
(Depressed + Right) + L3 (Depressed + Left). If the code is accepted you will 
hear a sound. Note: If you enable this code you cannot progress any further 
in the game. If you win a race it will not count.

Burnouts:
At any point during a race, hold R2 + L2 and press Circle. 
Release Circle and you should be doing a burnout. 
 
Best vehicle: Rally Car. 

Best terrain: Mud.

Blinding opponents:
Drive in front of an opponent while in mud to blind your him.
Now you can crash his car.

Extra boost at the end of a race:
When the finish line is fairly close, throw on the boost. Keep holding on 
the boost and the car's engine will began to heat up. Keep held the boost 
and your car will explode a few seconds later and you will fly forward 
at an extremely high speed to the finish line.