Find and store a couple armored trucks in your garage. They will respawn in your garage again when they are destroyed. Blow a truck up to get money from it. After collecting the money, return to your garage and get the truck again. It may cost $600 for repairs but you should get at least $1,000 for destroying one.
To easily gain money, rip out every parking meters and newspaper boxes you come across. They will give you a couple dollars each.