Press Forward while the countdown is commissioning, then hit the gas once "Go" appears.

Unlimited Shields and Turbos

While at the "Press Start" screen, press L + R, Z, L + R, Z.  This code is disabled after every race.

Unlock the StarCom Team

Beat the XG Career Mode.

Unlock the Extreme Lap Challenge

While you are at the main menu, press L, R, L, R, L, R, Z, L + R.

Unlock all Tracks and Teams

While at the "Press Start" screen, press L, L, R, R, Z, Z, L + R + Z.

Double All Prize Money

While at the main menu, press L, R, Z, L, R, Z, L + R.