Once you open up the ability to use "Bruski's Scrapyard", oh boy, it's big time MONEY!!! And unlike other "Get Rich Quick" scheme's, this one works!
Travel to Bruski's Scrapyard (at the top of the map, big "B"). Once you're there, drive the car you drove there in, into the car compactor. BaM! $400 something dollars. YOu'll notice some model car sitting there to the left of the compactor. Grab it, drive the track around the whole scrapyard, and when you see the compactor again, voila!!! a new car has taken the place of the one you just broke into and drove around the scrapyard in!!!!
This absolutely works every time, and you will see different models of cars show up after you drive around the track. That route was the shortest I could find, but technically you may not even have to drive that far. I used to drive farther and steal other cars, until I figured this out and then felt dumb and like I had wasted a bunch of time. Sure you can get other cars which pay more, but this way last easily under a minute. In no time you'll have many thousands of dollars. I'm at $50,000 something right now. HOPE YOU ENJOY!
p.s. forgot to add you can't compact cars that came with your garage, it won't except them, that and I can't get huge cars to fit into the compactor either