Black and Bruised Cheats

Getting 2nd costumes:
By finishing a characters LIFE MODE you will get their 2nd costume.

Every boxer:
Pick the CHEAT option in the setup menu and push Start, X, Circle, Square(2), R1(2), Square, Circle, X, Start.
User Cheat by GameGearJunkie

Invincibility:
Pick the CHEAT option in the setup menu and push Start, X(2), Circle(2), R1(2), Square(2), Start. 
User Cheat by GameGearJunkie

Turbo mode:
Pick the CHEAT option in the setup menu and push Start, R1(10), Start.

Constant power-ups:
Pick the CHEAT option in the setup menu and push Start, X, Circle, X, Circle, X, Circle, Square(3), Start.

Conversation mode:
Pick the CHEAT option in the setup menu and push Start, R1, X, Circle, Square, R1(3), Start.

Intercontinental Tournament:
Pick the CHEAT option in the setup menu and push Start, X(3), Circle(3), Square(3), Start. Or win all five default tournaments to unlock the Intercontinental Tournament.

Alternate costumes:
Pick the CHEAT option in the setup menu and push Start, X, R1, Circle, Square, Start. Or finish Life mode with a boxer to unlock a new costume for that character.

All boxers' Life mode:
Pick the CHEAT option in the setup menu and push Start, X, Square, Circle, R1, X, Square, Circle, R1, Start.

Scrap Yard arena:
Pick the CHEAT option in the setup menu and push Start, Circle, R1, Circle, R1, X(2), Start. Or by winning all survival modes to unlock the Scrapyard arena.

Unlock King Khan:
By winning the Top Contender Tournament.

Unlock Matador:
By winning the Intercontinental Tournament.

Unlock Odiva:
By winning the Pro Tournament.

Unlock Old Master:
By winning the World Championship Tournament.

Unlock Training Robot:
Finish training mode.

Restoring your energy:
Quickly push X repeatedly after getting knocked down to restore a small amount of energy and help prevent a knockout.