Unlimited shield:
Beat each level on Hard.

Unlimited ammunition:
Beat the game on Normal with an "A" rank on each level.

Selectable left-weapon:
Beat the game on Hard with an "A" rank on each level.

Unlimited grenades:
Beat the game on Hard with at least a "B" rank on each level.

Unlimited health:
Beat the game on Hard with at least a "C" rank on each level.

Thanks to Revolution readers Cheesy Poofs and tobias!