This is a ten step guide that will net you a crap load of money. When it was all said and done I ended up with roughly $6.3 Billion dollars between the three main characters which each having at least $2 Billion. I was able to purchase every property in the game and I have been buying all the clothes, planes, choppers, cars, guns, etc and still have at least $5 Billion left.
1. Lester's assassination missions are the number one key to this. DO NOT do them until you have beat the main story. There are five of them total and the first one you are required to complete in order to advance in the story but the remaining four...don't touch them. The one assassination mission that you will have to do you should make sure you put it off as long as you can. Eventually the game will tell you to do it in order to continue with one of the story heists so push it off until then. Also, do not wait until these missions begin to buy the stocks. Go all in with all three characters on the correct company before you approach the pay phone/lester (first mission is in person meeting).
2. Don't buy any properties or vehicles at all and don't waste money customizing cars you already own them until after you are done with the game...trust me on this. You want to build up as much cash as you can early in the game. Don't buy the hanger that they tell you to purchase with Trevor...leave it alone. Tattoos, clothes, haircuts, cab fares, etc are all fine. Keep the expenses to a minimum. You want to have as much money as possible on your characters early on in the game for this to work in full force.
3. When you do the first heist (jewelry store robbery) choose the crap drivers and shooters. You will likely lose some of the score because things will go wrong (I've heard you can stop and recover the lost duffel bag when the driver crashes) but it is worth it in the end. Every time you use someone they get 25% better at what they do but their minuscule 6-9% cuts stay the same. This way once you get to the final heist you can use people that aren't going to eat up a ton of the money but are now skilled enough to get the job done. You will get significantly more money on the last job ($32 MM roughly per character).
4 (NOT ALWAYS GUARANTEED). When you do the jewelry store heist mission the stock for that company (Vangelico) SHOULD sink big time to about $50-$90 a share (seems random). Go all in with all your characters when it drops because it SHOULD eventually climb back up and you will make a huge profit. The reason I stress the "should" is because this stock seems to be the most iffy of all of them. Some people report it never recovering or never dropping. I would give it a shot regardless but if you don't see it move much then get your money back. Also, his stock takes the longest to recover from my personal experience and the quickest way for it to go up is not to wait or do the repeated save trick but to complete actual missions. Since you shouldn't be buying anything until you get to the next step it shouldn't matter that your funds are in the VAG stock waiting to go up anyways.
5. When you get to the mission where you do finally have to do the first assassination mission (hotel assassination) spend every single cent you have on Betta Pharmaceutical stock before you even start the mission (so before you approach Lester on the bench). Wait for it to max out after the mission is over then sell it all. Then go all in with all your characters in Bilkinton while its down and wait for it to go back up to make another huge profit on the same mission.
6. Once you do the mission where you chase Molly through the airport (movie director mission for michael) the stock for FlyUS will crash due to the destruction of the planes. Go all in after the mission ends and wait for it to recover (might take a few in game days).
7. After you do the final heist...go all in with your characters on the Augury Insurance stock company. Once you do the main end mission after the heist the stock will soar and you will make bank (you will see this increase after the credits roll and you go back to your characters).
8. Not going to spoil the heist in case you haven't done it yet but you will make significantly more money if you take the guns a' blazin route vs the stealthy route because it requires less crew so less cuts to hand out (made $24 million on stealthy trial vs $32 million on the other option). Also, remember use the crap guys from the beginning that you should have stuck with. Their cut percentages will still be small but they will now be good members and experienced from being used previously. You should come out with about $33 MM per character here if you do it right.
9. Now that story is done...go back to the assassination missions and do as follows:
Assassination 2 (Multi): Go all in on Debonaire and then wait for it to spike up then sell it all and immediately go all in on Redwood. This is the biggest one..Redwood will recover and you end up around 300% profit.
Assassination 3 (Vice): Go all in on the Fruit stock beforehand and then cash out around 50% profit.
Assassination 4 (Bus): Don't buy anything up front. Once you are done go all in on Vapid. This takes the second longest to recover...I found that if you just die a few times it will pass a few days in game time and then it will go up around 100% so you basically double whatever you put in which at this point should be hundreds of millions.
Assassination 5 (Construction): Go all in on Gold Coast before you start the mission. Usually peaks around 81% profit.
10. For the kicker...invest everything you have into the stock called Tinkle with your characters and then find the man on the east side of the map about halfway up. He is a random encounter guy that asks you to take him to the airport and then once you get there he tips you off on the stock Tinkle and that you should buy it (you already should have). It will go up about 33% then sell it. Going into this encounter I already had about $1,600,000,000 from all the assassination stock payouts. This extra 33% pushed me up to over $2.2 Billion on Michael. The exact location for this guy can be found easily with a google image search.
This is all pretty much by the book what I did and I finished with over two billion dollars on each character when it was all said and done and between the three I had about $6.3 billion.
I should include that you deff want to do the Epsilon Program missions prior to doing all the assassination missions because you can easily net another $2MM to your total. Here is a guide: http://www.ign.com/wikis/gta-5/Seeking_the_Truth
Also...there are several random encounters with pedestrians that net you pretty good money. I suggest always checking in to see what they need especially if it's a bike being stolen or someone about to be killed. The wallet and purse robberies never get you anything so always keep the cash. There is also a weed farm northeast of Trevor's trailer home that you can rob once a week using Trevor only for $50-80K (seems random) and you can make $60 K for bringing back all of Maudes bounty targets alive. I would do these early in the game because you want to have large sums of money early on ($250K+) with each character because the way you gain money in this game is a snowball effect so the larger you start the larger you finish (stock market strategy).
Another last minute tip...the Sonar Dock station on the northwest portion of the map can be bought for $250,000 and then you can go around and grab the 30 nuclear waste objects with the submarine. You get $23K per pickup and then a $25K bonus when you finish all 30 so you end up making $715,000 when its all said and done minus the $250,000 you put into it so you get netting you a pretty huge profit ($465,000). It takes a solid 90 minutes to do though.