As you may have noticed, the game gives you the possibility to bet on horse-races, and ideally this happens relatively short after you started a new game.
There are two betting shops called "Inside Track" in San Andreas. The first one is directly located in Downtown Los Santos, the second one in Montgomery (Red County).

Overview:


"Inside Track" in Los Santos:


"Inside Track" in Montgomery:

Now, do the following:

1. Save your game.
2. Go ahead and buy a safehouse somewhere near the betting shop you prefer. Then save again. If you don't have enough money, ignore this step. You will soon have enough money to buy a safehouse.
3. Now, go to the "Inside Track".
4. Set ALL of your money on the horse that has the worst chances to win. (Look at the screen: the rate of 12/1 means that you will get twelve times the amount of money back if you win).
5. Wait until the race is over. If you win, take the money, go back to your mansion and save. If you lose, load your latest savegame and tray again. For this reason there is step 2: A safehouse near to a betting shop saves time to get there.

Important:

You can repeat this procedure as often as you want, but don't forget to save after you won, and always set the maximal amount of money that is possible!
I tried this out back in the days, and this method gave me the maximal amount of money within less than an hour.
In the end, you will never have financial problems in this game again!