Enter one of the following at the start screen:

Unlock GamePro t-shirt: DOWN, R3, UP, R3, L3, DOWN, L3, UP, DOWN, SQUARE
Unlock GameStar t-shirt: UP, R3, RIGHT, R3, R3, LEFTt, R3, DOWN, CIRCLE, CIRCLE
Unlock Harmonix t-shirt (Ishani): L3, CIRCLE, UP, CIRCLE, SQUARE, L3, DOWN, DOWN, R3.
Konami T-shirt (Dwayne): RIGHT, R3, RIGHT, R3, SQUARE, RIGHT, CIRCLE, SQUARE, DOWN, LEFT.
Game Informer t-shirt (Angela): DOWN, L3, L3, R3, R3, CIRCLE, RIGHT, SQUARE, L3, R3