Big Heads (ATVs/Bikes)
While in-game, press pause then hold L1+L2+R1+R2+R3 and hold right on the analog sticks, then add L3 while pushing left.

Unlock Everything
While on the main menu, hold L1+L2+R1+R2+R3 and push up on the analog sticks, then add L3 and push down.

Hint: Remove Mud and dirt off of screen
If there is mud and/or dirt on the screen, quickly press Triangle (2). This will clean the TV screen.

Hint: Extra Winning Boost
Use the following trick for an extra boost that can help you win at the end of a race. When the finish line is fairly close, make sure your boost meter is almost full, then use the boost. Do not stop holding on the boost and your car will begin to heat up. After the meter is all the way to the end, your car will explode, launching you a good distance forward at an extremely high speed.