Well, you can use glitchs (like a lot of people do, but it's obviously cheating lol) or you can constantly make well payed missions.
The classic well payed mission is Rooftop Rumble, but you'll need a friend or someone over lvl 75 or more so he can host if for you everytime you want to do it.
If you can't do that, then you have Coveted, which needs (i think) level 35.
Those missions used to be super easy, but with time rockstar made them harder and harder because people were making too much money from them lol
I got like $1M in the bank after having every car tuned because some time ago a weird mission appeared, it was super easy, lasted like 4 minutes and made you win $75k, I think it was called something like FOS_V3/V4, 2 days later rockstar removed it
