How is a Poke Ball turned into a Master Ball?

There is no known reliable way of turning a Poke Ball into a Master Ball without using cheat codes via a Gameshark or similar device. There are a number of claims that pressing buttons in the correct order when using a Poke Ball will change it into a Master Ball, but these have no effect on the chance of success. Master Balls are obtained through normal gameplay methods in most Pokemon versions, however.

Obtaining Master Balls Through Gameplay

• Pokemon Red/Blue/Green/Yellow/Fire Red/Leaf Green: The CEO of Silph Co. will award one to the player after defeating Giovanni in Saffron City.
• Pokemon Gold/Silver/Crystal/Heart Gold/Soul Silver: Professor Elm will award one to the player after obtaining the Dragon Fang from the Dragon's Den (Gold/Silver), or answering the Elder's test correctly (Crystal/Heart Gold/Soul Silver).
• Pokemon Ruby/Sapphire/Emerald: A Master Ball can be found in the Magma Hideout (Ruby), or the Aqua Hideout (Sapphire/Emerald), hidden between two electrodes and a nugget.
• Pokemon Diamond/Pearl/Platinum: Cyrus will reward the player with a Master Ball after being defeated at Team Galactic's HQ.
• Pokemon Black/White: Professor Juniper gives one to the player after they defeat Iris/Drayden in Opelucid City, provided they have traded 50 Pokemon through the GTS.
• Pokemon Black 2/White 2: After defeating the Elite Four, the player must defeat Colress on the Plasma Frigate. Professor Juniper will then award the player a Master Ball. Alternatively, one can be obtained at the Avenue Raffle Shops.
• Pokemon X and Y: The manager of the Pokeball factory will give one to the player once they have defeated Mable and a Team Flare grunt.