A) Many people still farm money by doing the "Pacific Standard" preparations and heist.
B) If you own a (biker) clubhouse you can do the respective missions from there. Some can be done solo while others will need at least one other clubmember. Most of them pay quite well.
C) The businesses you can own as president of an MC pay out quite well but you've to invest quite some money to make them really lucrative and have to spend a certain amount of time to collect the needed supplies to run them. It also takes some time until the "product" is manufactured and you've to be online during that period as the production will stop otherwise. The final delivery that pays you mostly needs one or two other players to be successful as you mainly have to deliver two or three vehicles to different locations (they can also be done alone but that's a little risky).
D) Collecting crates as CEO and deliver complete loads/badges also pays out quite well but you've to spend money for the warehouse(s) where the stuff gets stored and for every single crate you order before you'll make any profit.
E) Same for the new car dealing stuff. It seems to pay out quite well depending on what type of cars you collect but again you've to buy a garage where the cars can be stored in before and you also have to spend money on repairs (in case they'll get damaged) and upgrades (if ordered) of the collected cars.
F) Wait until you meet a "modder" and ask him to drop you some cash (not recommended as R* is monitoring this and may ban you at least temporarily).
G) Buy Shark Cards