International Superstar Soccer 64 Cheats & Tips

Extra Teams
At the Title screen, press Up, L, Up, L, Down, L, Down, L, Left, R, Right, R, Left, R, Right, R, B, A, then hold Z and press Start.

Big Heads
At the Title screen, press C-Up, C-Up, C-Down, C-Down, C-Left, C-Right, C-Left, C-Right, B, A, then hold Z and press Start.

Bonus Teams
Go to the Simulation mode where you are placed in a situation where your game has been started and you have to win or keep the lead given. Select difficulty 5 in the options menu. Beat all 16 and you will get new teams (which must be saved to the controller pack).