Enter the following two codes after defeating the first boss. The number of times each code can be used directly corresponds to the number of levels completed.

Double shot power-up:
Up, Up, Down, Down, Left, Right, Left, Right, L2, R2

Laser shot power-up:
Up, Up, Down, Down, Left, Right, Left, Right, L1, R1

Free play mode:
Play for 18 hours.

Weapon edit mode:
Beat the game, then go to the weapon selection screen and press R1. Press L1 to exit.

Thanks to Revolution readers Wind Jammer, Russel and Fire Kabong!