Whenever you view a boxer's intro movie it will be unlocked for viewing in gallery mode.
Lose 100 times in single-player mode to unlock headgear.
Beat Donkey Kong in Last Stand mode and he'll be available for exhibition mode.
Complete three challenges in exhibition mode on any character to unlock that character's audio gallery.
Regain Health Between Rounds
Between rounds you can press the minus (-) button to regain some of your health. You'll hear a sound if you've done it correctly.
When you're just about TKO'd press the 1 and 2 buttons repeatedly. You'll get another chance to fight, but this only works once per fight.
Regain health while your opponent is down
When your opponent is knocked down you can alternate tapping the 1 and 2 buttons to regain some health.