All characters:
Up, Square, Right, Circle,
Square, Left, Circle, Up, L3,
R3
All costumes:
Up, Circle, Left, Square, Square,
Down, Circle, Right, R3, L3.
All venues:
Square, Triangle, Right,
Up, Left, Down, R2, L2, Start
GamePro T-shirt:
Down, R3, Up, R3,
L3, Down, L3, Up, Down, Square
Game Star T-shirt:
Up, R3, Right, R3, R3,
Left, R3, Down, Circle, Circle
Game Informer T-shirt for Angela:
Down, L3, L3, R3, R3,Circle,
Right, Square, L3, R3
Harmonix t-shirt for Ishani:
L3, Circle, Up, Circle,
Square, L3, Down, Down, R3
Konami T-shirt for Dwayne:
Right, R3, Right, R3,
Square, Right, Circle, Square, Down,
Left.
PSM T-shirt for DeVron:
Left, Right, Left, L3,
R3, Down, Up, Up, Circle, Square
Enter the following at the 'Cheat' screen:
Banana mic:
L1, L1, R2, R2, Right, Down,
Circle, Left, Up, Square
Toothbrush mic:
R1, L1, R2, L2, Right,
Left, Down, Up, Circle, Square
Doll mic:
Circle, R2, Square, L1,
R1, L2, Square, Up, Circle,
Square
Big head:
Down, Down, Down, Up, R1, L2,
R2, L1, Circle, Circle.
Small head:
Right, Right, Up, Up, L2, L2,
R2, R1, L1
Big eye:
Square, Square, Square, Square, Circle, Down, Down,
R2, L2, R1
Glass character:
Square, Square, Circle, R2, R2,
L2, Down, Right, Right, Up
Oil slick character:
L2, R2, L2, R2, Square,
Down, Circle, Up, Left, Right
Mercury character:
Circle, L1, R2, Up, Up,
Left, Left, Square, Square, R1
Wraith character:
R2, Left, R1, Right,
Up, Up, Square, Circle, Down, L2
Tough crowd:
Right, Up, Circle, Circle, Circle, R2,
R1, R1, L2, Down
Pirate crowd:
Square, Square, Left, Left, R2,
L1, R2, R1, Circle, L2
Robot crowd:
Circle, Circle, Right, Right, R2,
R1, R1, R2, Circle, R2
Zombie crowd:
Left, L1, L2, Square, Square, L2, L2, L1, Left, Left.
Thanks to Revolution readers Genius and dancefan!