Press START and tap the D-Pad for:

Low Gravity - UP, RIGHT, RIGHT, UP, UP, LEFT
No Target Mode - LEFT, RIGHT, RIGHT, LEFT, UP, DOWN
IFF Glow - RIGHT, RIGHT, RIGHT, UP, DOWN, LEFT
Big Heads - UP, UP, LEFT, RIGHT, UP, DOWN
Player clipping - UP, DOWN, LEFT, RIGHT, DOWN, UP
Autoaim Toggle - DOWN, DOWN, DOWN, DOWN, UP, UP
Add 16,000 Credits - UP, UP, DOWN, DOWN, LEFT, RIGHT
Outfit Change - LEFT, RIGHT, DOWN, UP, LEFT, RIGHT

Note: These only work for single player games (i.e., not Xbox Live).