story mode

When you do the assination missions. Invest in whoever their competitor is before killing them and there stock price will fly up. If you listen to what lester says when your doing the missions you might not have to look up the exact missions and companies to invest in.

There are a lot of trainers you can use, I forget their names, but there are a lot of good ones.

Tons of trainers that work . Just be careful using them online lol