Paintball mode:
Press Right, Left, Right, Left, Down, Down, Up, Up.

Every multiplayer skin:
Press Down, Left, Up, Left, Right, Down, Left, Up.

One life mode:
Beat the game on hard.

All Artworks:
At the extras menu, press Down, Right, Down, Right, Down, Up, Down, Down, Right, Down, Right, Down, Up, Down, Left.

All campaign & multiplayer modes:
At the extras menu, press Down, Right, Down, Right, Up, Down, Up, Left.

Restore Armor and Health:
While playing, press R, R, R, L, R, R, L, R.