Complete the indicated tasks to unlock the corresponding costume.

Gym Uniform: Complete Chapter 3.
Bunny Girl: Complete Chapter 4.
Stage/Rocker Costume: Complete Chapter 8.
Swimsuits: Complete Chapter 11.
Nurse Outfit: Collect all the Mikuru Wiimote Voices.
Cheergirl: Complete Chapter 7.
Bunny Girl (alternate colors): Play through the game a second time.
Swimsuits (animal ears): Play through the game a second time.
School Festival Waitress/Maid: Get an "S" rank on all stages in Story mode.