Go to Bruski's Scrapyard. There is a random car located next to the car
crusher. Break into the random car. You do not need to pick the lock
because there are no cops nearby. Drive the car into the car crusher,
and crush it to get approximately $400. Get in the car you came in, and
drive it out of the scrap yard using the north exit located on the map.
Travel this road until you go under a bridge. Then, turn around and go
back to the scrap yard, and another random car will have respawned.
Repeat this as many times as desired. You can also easily get the
"Proper Scrapper" trophy using this method.

Go to any 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
without spending money. 
Note: If you are in desperate need of money, do this at a gun store.