The main way I make money is by doing the mission Trash Talk, I always activate this mission from my garage otherwise it starts you at Madrazo's place which I hate driving from. Once started I phone for my Buzzard then fly round and destroy the trash trucks (takes about a minute to do,) fly to the scrapyard and destroy the two grangers parked on the hill behind first of all. Then take out the crew and trucks but don't destroy the second truck into the compound, the one that's parked by itself in the shed to the right next to a gas tank.
After i've done that and the one truck is remaining I phone for my Tornado, go down to Vagos and Ballas turf and drive backwards and forwards between the two waiting for a gang version to spawn (during the mission you can't get wanted levels, bounties put on your head for stealing cars and gang members won't attack you.) During this time I also fuck about to kill time, I often go on a hatchet rampage against the Vagos who are powerless to do anything or fight back. Once you've wasted enough time to get the maximum payout and you've got a gang Tornado drive it back up to the scrapyard, put one bullet into the gas tank destroying the truck, then kill the gang boss to complete the mission for $22,860. I then put it into passive mode while I drive the Tornado to LS customs (you never know when some fucker in a Hydra will come by and blow you up,) sell it for $13,575 and bank the profit. Total is about $35,000, but naturally you can only sell the Tornado once every 48 minutes.
Another good way to get money is by taking down the smuggler planes/helicopters, the profit you get is incremental and starts off at $2-3000, but it caps out at $30,000. A lot of people don't seem to bother going for them, which suits me fine. While waiting for them I camp on top of the IAA building with my heavy sniper and Buzzard at the ready, taking potshots at twats flying about in Lazers/Hydras causing havok.