Knockout Kings 2000 [N64]

Easy knock:
Hit your opponent until your meter is full and then hit R + L + KICK (A or B) and you will knock him down even when he has his block on.

Easy training:
In the training mode go to OFFENSE and hit the blue guy like this:
DOWN + A (twice)
DOWN + B (twice)
I think it's the only way to hit him 30 times.

Invincible mode:
Pause the game and press C-Up, C-Up, C-Left, C-Down, C-Right.

Big Gloves:
Pause the game and press C-Up, C-Down, C-Up, C-Up, C-Down. Repeat to de-activate.

Big head mode:
Pause the game and press C-Left, C-Right, C-Left, C-Left, C-Right. Repeat to de-activate.

Restore energy:
Move away from your opponent, hold Z and sweep the Analog-stick in circles. Also try tapping Z repeatedly without moving the Analog-stick.

Easy attribute points:
Select career mode and intentionally lose every fight so you can train to earn more attribute points. Once this has been done ten to fifteen times, all your attribute points should be at the top. This will allow you to easily win your remaining matches, and earn the championship after about five fights. Note: Every time you lose a fight, you can train again.