Note: May only work on the unpatched version of the game. Go into the restricted area at the Los Santos International Airport through the fence (jump it or drive through it), near the flight school to find three helicopters. Use the helicopters to escape the police after robbing stores, but make sure the helicopter is not parked where the cashier can shoot you. Robbing and escaping the cops will also earn you easy XP. You will usually get from $1,000 to $8,000 for each robbery.