Pro Action Replay Codes
Since the Pro Action Replay cartridge has the capability to write to memory addresses outside of the Game Genie's range, the following codes may only be enabled with that device. It is not possible to convert them for use on the Game Genie.
Note: The codes work in both parts 1 and 2.
To gain more stats, enable the code, increase your level until 22 after a battle, then stop. Disable all codes, go to an inn and sleep. Your level will return to 2, but all stats will remain high. Enable the codes again, enter a battle and repeat.
All codes above from Paul.