Go to the main menu and hold R+L. Now you can use the following cheat combos:
Unlimited Combo Meter: Left, Right, Down, Up, Up, Down, Right, Left
All Upgrades: Down, Up, Down, Left, Down, Right, Up, Down
Unlimited Health: Up, Right, Up, Left, Down, Left, Down, Right
Dark Knight mode: Right, Up, Up, Right, Left, Down, Right, Up
All Rewards: Left, Down, Left, Right, Left, Left, Down, Right