In terms of the day to day... As in real life, buy low & sell high! To go more in depth, check low stocks that are starting to rise a solid amount. Move quickly on your money once they start to dip if you have made a decent enough investment.
Then there are the assassination missions from Lester which have SERIOUS end game benefits if you wait to invest all your career money(should be 40m if you play the heist right) in certain business before/after the missions. This will result in you ending up with 1.5 Billion Dollars for Michael, Franklin & Trevor.
Precisely from USgamer.net:
1. Finished the game
2. Maximum amount of money (35 Million) from the big one. (Requires you to take the Obvious approach for less team members used, also use all of the lowest costing heist members. Will need to level them up wisely to do)
3. All of Lester's Assassination missions left untouched (Aside from the first one which is mandatory)
Step 1: Invest in Debonaire before the mission. After the mission, sell all of Debonaire when it reaches maximum profit and then INVEST in Redwood!!!! My Return= 300% (Editor's note: You know a man's serious when he uses that many exclamations marks.)
Step 2: Invest in Fruit shares before the mission and cashout afterwards! My Return= 50%
Step 3: Invest in Vapid AFTER you complete the mission. Cash out when you made your cut! My Return= 100%
Step 4: Invest in Gold Coast Development before the mission and pullout afterwards! My Return= 80%
Step 5: Trigger the Hitch Lift 1 random event (Location: Ineseno Road, Banham Canyon) and complete the event. He will then give you a stock tip about the company "Tinkle", invest for Return = 30%