Level skip:
During play, pause the game and hold L1 + R2 + Down + . Release those buttons, then quickly press , Up, , , Right, Down. A level skip option should appear at the pause menu.

Unlimited items:
Money and items respawn, so just find an area that you can safely exit and re-enter, and you can keep collecting whatever is there.

Thanks to Revolution readers Michael McEleney, Chunky Of The Chunky and Tiny Mafia!