After the mission where you must dump Mario's body, you will find yourself at a casino. There are two pieces of loot you can collect: a cell phone near the slot machines, and more importantly some credit cards over at the bar. The cell phone is only worth a few hundred dollars, but the credit cards are worth almost $1,000. After collecting these items, use a slot machine. Exit out of it as soon as the slots pop up. You will find yourself back in the casino, and the cell phone and credit cards will be back in their original locations. Keep collecting them repeatedly to get as much money as desired.