For a quick $5,000, watch for armored cars to appear. They are marked by blue dots on the map. Simply kill the guard before he enters the armored car. If you have to open the armored car to get the money, drive into water to pop open the rear doors.
Keep an eye on rivalries between large corporations; Lester will give you information about which companies are rivals. Since destroying the assets of a corporation will cause their rival's stock to rise, use it to your advantage by first investing in the rival's stock. For instance, destroying planes belonging to AirEmu at the Los Santos Airport will cause stocks in FlyUS to rise. The same can be done with oil derricks belonging to RonOil or LTDOil. When a mission objective includes destroying a corporation's asset or reputation, make sure you first invest in their competitor.
Once you have two playable characters, go to Paleto Bay and swim into the ocean. Dive at the indicated location and collect the $12,000 from the crashed airplane. Once done, switch characters and switch back to your original character. This will respawn the underwater money, as well as restore your oxygen. Collect the money again and repeat the process.
Take a sub and search the seafloor of the western coasts until you find a structure that resembles a sub, only much larger. You can find an RPG on the seafloor next to it, and a briefcase containing $25,000 dollars laying on top next to the open hatch.