Lizard team:

Select exhibition mode and choose Kaufmann Stadium. Then while playing a game, hit one of the signs that say "Win a lizard" to transform all players on the team into lizards.

Unhittable fastball:

Select arcade mode. Then while pitching, hold C-Down + A until the ball reaches the plate to throw an unhittable fastball.

Faster running:

Hold Z to run faster.

Invisible baserunners:

To get invisible baserunners, when you have a man
on base, hold the A button for 30 seconds, then hold the up button for 13 seconds, then press A, B, Up, Down, Start.