All stars:
Press R, Up, Down, L, X, B, Left, Right, B, A on controller two at the title screen.

Dogs:
Press Up(2), Down(2), Left, Right, Left, Right, B, A on controller two at the title screen.

Turn referee into a dog (Japanese version):
Press Up(2), Down(2), Left, Right, Left, Right, A, B before the Japanese introduction is spoken. The sound of a dog barking will confirm correct code entry.

Extra skill points:
Enter edit players mode before each match and use all available skill points. Highlight the "Cancel" option and press Y for 200 additional skill points.