How to Get Unlimited Master Balls in Pokémon Gold/Silver

Obtain the Master Ball from Professor Elm in New Bark Town after defeating the Johto Gyms.

Give any Pokémon (preferably a useless one) your Master Ball (keep in mind that if you do this incorrectly, you'll lose that Pokémon).

Go to Bill's PC and deposit the Pokémon in a new box that is empty.

Swap the box for another.

Once it says "Saving...Do not" shut the game off. What happens is that it saved the Pokémon in the box, but you turned it off before it took it out of your party.

Turn the game back on and voilà! You have another Master Ball and a clone of the Pokémon, one in your party and one in the PC.