Go to Bruski's Scrapyard and break into the random car located near the car crusher. Drive the car into the car crusher and crush it to get about $400. Enter the car you came in and drive it out of the scrap yard through the north exit. Stay on this road until you drive under a bridge. Then, turn around and return to the scrap yard. Another random car will have respawned near the car crusher. Repeat as desired. This will also unlock "Proper Scrapper" trophy.
Go to a store and spend some money, then pull out a gun and rob the store. You will get more money than you would by just robbing the store alone.
You will earn a trophy named "Hey Joe" when you crush the car that has the bartender's body in it after Joe killed him. In the Jimi Hendrix song "Hey Joe", the lyrics include the line "Hey Joe, where you going with that gun in your hand."
When you go to prison and to the yard, there will be a crazy man in the center. He says phrases such as "All your base are belong to us", "Someone set us up the bomb", and "You have no chance to survive make you time", from Zero Wing. If you talk to him, he will run away.
Note: do this in either Jimmy's Vendetta or The Betrayal of Jimmy. First, start the mission with the vehicle you would like to keep. Once you are inside the vehicle, get back out and get into another car (preferably the one you started the mission with). Get back out and into the target vehicle once more. At this point, complete the mission. At the scoring screen, select to REPLAY the mission (not continue). The replay should start with the target car as your vehicle. Drive it to a garage, store it, and then complete the mission again. Now you have the vehicle in your garage, and can use it as you like! This also allows you to acquire cars that are impossible to find elsewhere, such as the Culver Empire in the Betrayal of Jimmy and many 1940's-only cars in Jimmy's Vendetta, as well as the Prison Bus (although the bus can't be stored in a garage).