Cheat mode:
Press Up, Right, Down, Left, 1, 2, 1, 2, Start at the title screen. Level select and other options will be available.

Invincibility:
Enable the "Cheat mode" code. Highlight the "Mode" option and press Left, 1 + 2.

Easy money:
Shoot the hamburgers within the first thirty seconds of starting level one for 1000 gold per hamburger.