the biggest money you can make is by investing in a competitor just before you ruin a company or its leader before a mission. Thing is, you need to know who is who's competitor.
The slower option is to invest in stock that has recently gone down A LOT and buying it with the hope it can only grow from that point onwards.
Lester can give you some pointers at times, (it is suggested he makes most his money doing this) but he isn't always as forthcoming with his info as he should. The missions he gives to Franklin are your best bet. Save all your money up to those points. Invest in the known competitor of the company owner you are going to assassinate, and after the mission you can sell again for a lot of profit. You don't even only have to invest with Franklin either.
I am sure somebody will put up a guide soon, since this is definitely the fastest way to make money in this game.
so if the investment doesnt work out, reload from a different save?
With either quick save via phone (apparently saves your EXACT position as well) and regular saving (sleeping) you can save in any slot you want, just test out loading a quick save and such and see how it works.
Forgive my ignorance, but how do you make money by investing in a company before you ruin them? Wouldn't that cause the stock to fall way below the price you bought them for?
I also invented in LifeInvader before... you know, and now it's worthless. Is there any chance at all that it can go up?
You are misinterpreting. Say you know you are going to ruin LifeInvader. So invest in their competition. Once LifeInvader is ruined, you can then sell your shares you bought from their competition for maximum profit. Because now that LifeInvader is ruined, their competition now has the top spot and their shares will have increased in $$ value.