Enter the following as Gym names in Championship Mode:

CHAMP    Enable Championship class boxers
GOLD     Enable Gold class boxers
SILVER   Enable Silver class boxers
BRONZE   Enable Bronze class boxers

Cheap training:
In Training, highlight the Rumble Aerobics Training and press Left and then quickly press A. If you did it fast enough, you will buy the Rumble Mass Nutrition Regime (which costs $25,000) for $500! This will boost your strength and experience very quickly.

Change Costume:
Press C-Up + C-Left to change the outfit and the pose of the boxer.

Life when opponet is down:
When you knock you opponent down while the ref is counting tap L + R and your health will refill

Thanks to Revolution readers Adam Borchert, Troy, Alex Yo Juggalo Homie, yamcha (who is really justin, so there), and Markie Doolittle!