Free play:
At the Opening screen, hold L1+L2+R1+R2 and press Start. Keep the buttons held and press Up, Up, Up, Up, Up, Up, Up, Down, Up, Up, Up, Up, Start. You'll hear a sound to confirm entry. Now go to the 'Configuration' screen to toggle credits and level select.

GameShark Codes

Infinite Lives P1		800d784c0005

Infinite Special Attacks P1	800d787a1000

Invincibility P1		800d9c080095                                                          

Thanks to Revolution readers David LaForest, Colemicheals, and PKaopua!