Find an ATM
Load a saved game on your PS3 or begin a new game. The difficulty level doesn't matter; what does matter is that you're in the city and you have no wanted level. Look on your map to locate an ATM, or run or drive to Liberty City's financial district in the heart of downtown. Several ATMs are located here.
Traffic Jam
Choose an ATM near a two-way, three-way or four-way intersection and obtain a larger vehicle such as a coupe, taxi, cargo van or delivery truck --- either pick a saved one up at the garage or, in true "GTA "style, steal one. Make sure when take the vehicle that the driver runs away in terror; otherwise, and angry driver will confront you and try to engage in a slugfest that'll draw unwanted attention from the cops. If this happens, run and hide until the coast is clear and return to the spot.
Grab and Go
Once you have snagged the right vehicle, drive it into the intersection to block the flow of traffic. Pull the vehicle forward to cover all paths of escape from the roads in the intersection. Wait near the ATM for a customer to approach it; after she gets her cash and walks starts to walk away, mug her.
Repeat Often
Gather the money quickly and run. Blocking the area prevents patrol cars and ambulances from nabbing you or reviving the victim. Should the money not immediately spawn or a cop on foot gives chase, return once you've lost him to recover the money. This glitch method of gaining quick cash can be repeated ad nauseam throughout the game without fail, as the game automatically re-spawns customers at the ATM.