Circuit Breakers

Better Engines (2 Players)
Press X+Square and Triangle+Circle before selecting a track.

Jumping Cars
When the race starts to count down, press and hold <+Circle.

Night Driving
While selecting a track, press and hold L1+L2+R1+R2.

Reverse Tracks (2 Players)
At the circuit select screen, choose the circuit and press Select.

Track Select
In single player mode, pause, enter the sound option, highlight FX Volume, and press L1+L2.

Upside Down Tracks 
Before picking a track, press L2+R2 and X+Down.