2. Do crimes: This is self explanatory really, Once your high enough to do all the crimes do them all together and you will save up a bit of money
3. Gamble: Gamble your money on Blackjack or maybe roulette? Here's a little tip which sometimes works. Bet $1 on blackjack and try to win, Bet another $1 and lose, then bet one more dollar and lose. Then bet a load of money. I would say 80% of the time this works, but you can only do it a couple of times. Normally i get two kings after I've done the 3 dollars. I hope this works for you :)
4. Hustle cars: This is my favourite and it the one which got me most of my money, Buying and selling cars. If you have a vast ammount of money ($20,000,000 & Over) you should buy cars (Ferrari Enzo, Porche Carrera GT and all rare cares such as the Lamborghini Gallardo) and then sell them for more! I've done this a lot of times, I normally buy Enzos for 25-35mil, and then i sell them for 40-50mil. Its a great profit each time. It depends on the car and the ammount of cars there (Including their prices)
5. Busting: Busting people out of jail can get you a considerable ammount of cash but it also backfires if you fail to bust. Busting will take a long time to master especially if you are a low rank. The way to be able to bust.. is to just keep trying! Also if you have killed Paulie and Norman you are able to bust Sam out of jail for $2,500,000. I think i have made well over 10mil through busting so i hope it works for you!
I hope you find these money making techniques helpful!