If the stock price goes down then reload your initial save game and try again.
If the stock price goes up, then save your game and repeat the steps above.Provided you stay on the 'purchase more stock page' the price will constantly refresh for you. Once saved then check the stock by pretending to purchase more.Don't muck around when doing this (you will have 45 seconds or less). Buy it (the maximum amount) and then immediately save your game.Look for the stock that has the lowest price (meaning you can buy more of it).You should also know that the prices of the LCN stocks will refresh every 45 seconds while you're doing this. For this reason it is good to do after you've finished all of Lester's assassination missions, for example. This only works for LCN stocks and do make it worthwhile you need A LOT of money.
We have a page dedicated to Lester's Assassination Missions here. Basically, it boils down buying and sell specific stocks before and after you complete the assassination missions for Lester (which are a big part of the single player story line).
The 'Lester Assassination Missions' method of investing in GTA 5's stock market is probably the most fool-proof method to make money in in the game, and lots of it too.
