During a mission press start to pause the game, choose In-Game options, choose Enter Cheats, then enter one or more of the following button combinations:

Code - Effect

B, B, X, Y - Complete Current Mission
B, B, X, B - Team Superman (Team Invinvibility)
B, B, X, X - Refill Ammo
B, B, X, A - Superman (Player Invincibility)