When selecting a song, press Option (instead of A button).
Instead of the normal symbols scrolling on the screen (A, B, Arrows), it will instead be Fishes, Honey Jars (or something like it), and Mice (respectively). Also, Felicia will be in her other costume.
On the Character Select Screen, Press and hold A on the character you select to get a new costume color.
To access blood in the game, simply change the language to Japanese in the internal settings of the NGPC.
Successfully complete game on Gamer difficulty.
After choosing either Tag or Team mode, select one character (or even two, if playing in Team mode). Afterwards, press the option button, and you'll play down a man (or two).