Unlock Alternate Costumes:
This is done by pressing Up or Down when creating a custom player.

Unlock Bonus Courts
When you have selected your players at the 'Choose Team' screen enter one of these button combinations to unlock the court you want to play.

Team 1 (Left):
Hold Up + Turbo

Team 2 (Right):
Hold Down + Turbo

Unlock NBC Court:
Hold Down + Shoot + Pass

Unlock Street Court:
Hold Left + Turbo

Unlock Island Court:
Hold Right + Turbo

Unlock Midway Court:
Hold Up + Shoot + Pass