All Items  Pause and press R1 (5x), L2 (5x).

All Keys  Pause and press Up, >, Up, <, Down, Up, >, <, >, Down.

All Weapons  Pause and press L1, L2, Up, L1, L2, Down, R1, >, R2, <.

Big Head Duke  Pause and press R1 (9x), Up.

Big Head Enemies  Pause and press R1 (9x), L1.

Credits  Pause and press Select, Up (9x), Select, R1.

Double Damage  Pause and press L2, R2, L2, R2, L2, R2, L2, R2, L2, R2.

Ending  Pause and press Select, Up (9x), Select, L2.

Game Completed  During gameplay, press and hold L2+R2 and press Square, Triangle, Circle, Circle, Start.

Infinite Ammo  Pause and press <, >, <, >, Select, <, >, <, >, Select.

Infinite Continues  Pause and press <, >, Up, <, >, Down, <, >, L1, R1.

Intro Sequence  Pause and press Select, Up (9x), Select, L1.

Invincibility  Pause and press L2, R1, L1, R2, Up, Down, Up, Down, Select, Select.

Invisibility  Pause and press L1, R1, L1, R1, L1, R1, L1, R1, L1, R1.

Kilt Sequence  Pause and press Select, Up (9x), Select, R2.

Level Select  Pause and press Down (9x), Up. Then, quit the game, go to the main menu, choose your level with < or >, and press X. 

Rogue Trip Video  At the GT screen, press and hold L1+L2+R1+R2.

Temporary Invulnerability  Pause and press R1, L2, L1, L2, R1, L1, R1, L2, L1, L2.

Tiny Head Duke  Pause and press R1 (9x), Down.

Tiny Head Enemies  Pause and press R1 (9x), >, or pause and press R1, R1, R1, L1, L1, L1, R2, R2, R2, >.

(Note: enter the codes where you need to pause at a slow and even pace--don't enter them too fast.)