Instructions
Defeat the Petalburg gym leader, who happens to be your father.
Return to your house and speak with your mother. She will ask you whether you defeated your father, then give you the ̶0;Amulet Coin̶1; item.
Equip one of your stronger Pokemon with the item and put him in your first battle slot.
Fight the reporters or gym leaders and make sure the Pokemon wearing the Amulet Coin takes part in the battle. You will earn double the money you normally would win.
Continue to win battles using the Amulet Coin throughout the game. As you gain more powerful Pokemon, you can equip them with the item. Over time, just from regular battles, you will earn a lot of easy extra money.