All characters:
Up, , Right, , , Left, , Up, L3, R3

All costumes:
Up, , Left, , , Down, , Right, R3, L3.

All venues:
, , Right, Up, Left, Down, R2, L2, Start

GamePro T-shirt:
Down, R3, Up, R3, L3, Down, L3, Up, Down,

Game Star T-shirt:
Up, R3, Right, R3, R3, Left, R3, Down, ,

Game Informer T-shirt for Angela:
Down, L3, L3, R3, R3,, Right, , L3, R3

Harmonix t-shirt for Ishani:
L3, , Up, , , L3, Down, Down, R3

Konami T-shirt for Dwayne:
Right, R3, Right, R3, , Right, , , Down, Left.

PSM T-shirt for DeVron:
Left, Right, Left, L3, R3, Down, Up, Up, ,

Enter the following at the 'Cheat' screen:

Banana mic:
L1, L1, R2, R2, Right, Down, , Left, Up,

Toothbrush mic:
R1, L1, R2, L2, Right, Left, Down, Up, ,

Doll mic:
Circle, R2, , L1, R1, L2, , Up, , Square

Big head:
Down, Down, Down, Up, R1, L2, R2, L1, , .

Small head:
Right, Right, Up, Up, L2, L2, R2, R1, L1

Big eye:
Square, , , , , Down, Down, R2, L2, R1

Glass character:
Square, , , R2, R2, L2, Down, Right, Right, Up

Oil slick character:
L2, R2, L2, R2, , Down, , Up, Left, Right

Mercury character:
Circle, L1, R2, Up, Up, Left, Left, , , R1

Wraith character:
R2, Left, R1, Right, Up, Up, , , Down, L2

Tough crowd:
Right, Up, , , , R2, R1, R1, L2, Down

Pirate crowd:
Square, , Left, Left, R2, L1, R2, R1, , L2

Robot crowd:
Circle, , Right, Right, R2, R1, R1, R2, , R2

Zombie crowd:
Left, L1, L2, , , L2, L2, L1, Left, Left.

Thanks to Revolution readers Genius and dancefan!