Alternate costumes
Press (C-Up) + (C-Left) at the character selection screen to change the boxer's outfit and pose.

Flurry mode
Spell out RUMBLE, press (A) + (B), and when your opponent is close press (C-Down) + (C-Right).

Fight with Tank Thrasher
Quickly and repeatedly press (C-Up) + (C-Down) + (C-Left) + (C-Right) until Tank starts to make fun of his opponent.

Taunts
Press (C-Left) + (C-Down) or (C-Up) + (C-Right) during a match to perform a taunt.

Champ class boxers
Enter championship mode and enter "CHAMP" as a gym name. Exit championship mode and enter arcade mode to have access to all the boxers including Damien Black.

Gold class boxers
Enter championship mode and enter "GOLD" as a gym name. Note: This also unlocks Nat Daddy in arcade mode.

Silver class boxers
Enter championship mode and enter "SILVER" as a gym name. Note: This also unlocks Bruce Blade in arcade mode.

Bronze class boxers
Enter championship mode and enter "BRONZE" as a gym name. Note: This also unlocks Kemo Claw in arcade mode.