At the Game Start/Option screen, press X, A, L, R, Left, then hold L and press X, C, Z, A, Right, and Right. You will hear a sound if done correctly. Start the game and you'll notice that both players have nine credits instead of three.
Do the following tricks at the Game Start/Options screen.
Very easy/Abnormal modes: Press and hold X, and press Z, C, L, B, Left, R, L
Auto rapid fire mode: Press and hold B, and press Y, Right, Left, X, Z, L and R
Nine Credits: Press X, A, L, R, Left, then press and hold L, and press X, C, Z, A, Right, Right