Nick a Car

My personal favourite. Good cars mean money. Crappy cars mean bullets. Any cars mean good RP and no cars mean a little increase in RP too. So there's no bad results when you steal cars, just do it every time you can. Pick the option with highest percentage and hope for the best.

Crusher:
How do crappy cars become bullets. Very simple: Crusher.
A crusher is something families hire. The crushers can be of different sizes and qualities. Normally families hire crushers that can crush up to 3000 cars (total, NOT per user) and turn every car you crush into 12 bullets.
How to use it: Make sure the cars you wanna crush are in the same city as you, or that you're in the same city as them. Go to your garage page, check all cars you wanna crush and scroll down. There's a key called Crush: It'll crush all the selected cars- Free bullets! \o/

Advice: Ship all your crappy cars to one city, a bit before the family hires a cruser (you'll be warned, normally), fly to that city so you can crush them all quickly. You'd be amazed at how fast a big family can crush 3000 cars and fill a crusher.