Instructions
Run into a bubble containing a short sleeved shirt to collect it. Move the left analog stick on the PlayStation 3 or the directional pad on the PlayStation Portable to walk or run. Once you complete the level you are playing, the new shirt will be available in the Popit menu, which is where you customize your "sack person's" attire.
Press the square button to open the Popit menu.
Scroll to "Customize Character" and press "X." You will now be able to edit your attire.
Press "R1" and "L1" on the PlayStation 3 or "R" and "L" on the PlayStation Portable to navigate the pages of attire. Scroll to the "Torso" section, which contains shirts.
Scroll through the available shirts and highlight the short sleeved shirt you want your "sack person" to wear. Press "X" to put on the shirt. Press "X" once more to remove it.