Hold L+R and use the following key combinations after you pause the game:
Unlimited Missile Weapons: B, X, A, Y
Ammo: A, Down, Y, Up
Health: Y, Down, A, Up
1000 Experience: A, Down, Down, Down
Level 2 Skills: X, Right, X, Right
Level 4 Skills: Y, Up, Y, Up
Level 6 Skills: B, Left, B, Left
Level 8 Skills: A, A, Down, Down
Slow Motion: Y, X, A, B
Tiny Enemies: Y, Y, A, A
Invincibility: Y, B, A, X
Devastating Attacks: B, B, X, X