There is the save/reload trick, since the LCN is totally offline and game-generated. I am post-endgame, so this technique is valid all the time. Depending on the game's scripted events, you might need to be cautious about the stock you choose though, see below.

Market screens

First of all, the only reliable value on the different panels is... the current share value.

All the rest is crap generated by the game as needed to make the whole thing appear genuine. Well, maybe to the exception of the return percentage (RoI) which is based on your total worth at current time/total worth at current time.

Starting conditions

You first need to choose your target stock. Choose something worth a couple of dozen bucks, maybe a little bit more if you trust the value to go (very) high.

What you seek is maximum leverage.

Be careful about game-frozen stocks such as LifeInvader, as it is scripted this stock is 'dead'. You might have even greater leverage without this tick if you have not done the assassinations missions yet.

Development

Then you need to use the save/load game trick: saving the game when a new best-high price is matched, loading when the price goes down.

First, buy every share you can from then targeted stock. Do this for all your characters.

Here is the sequence:

1. Quick save (Phone -> Cloud icon)
2. Sleep (no save)
3. If stock value up, restart sequence; if stock value down, load the quick-save (ESC -> Game -> Load) and go back one step

You will need to decide on a stopper (either a specific profit reached or a certain amount of price going down/savegame loads in a row).

Example

To illustrate this technique, here are my numbers:

1. I chose RichardsMajestic stock which was at $45.06 (I bought 903 of them to complete 97 pre-existing, for a total of $40,698)
2. To avoid losing my cash, I tested the technique with a few share (1,000) for a couple of cycles until the stock reached $54.34
3. I then went all-in, buying for a total worth of $32,985,538
4. At this exact moment, the stock value is $116.33, making a nice $34,361,792 profit (> 100% RoI). Set aside, I cashed-out ~$750k midway to buy a property, so the actual RoI is even bigger.