Extra 30 Lives
At the game's start or after the game over screen, at the title menu, press Select 20 times then press Start to continue the game with 30 lives. This only works if you continue from the first two levels.
Note that this cheat only works in certain versions of the game dubbed "Rev. 1.X". At the title menu, press Select 10 times, then press A to switch between levels, then press Start to play from the selected level.
When you start the game, go to 'create'. From there, make three characters and put them into the group. Take the new characters' gold, weapons, and armor, and give them to the old character, then save. Restart the game and erase the new characters. Repeat until you're filthy rich.