What is the fastest way to get money in GTA San Andreas?

Running taxi missions can get you a small nest egg, which you then want to invest in the horse races - the odds of winning are pretty unrealistic. I seem to remember it being the same odds of success regardless of the odds on the horse.

Save your game before you start, and then bet the max you can on the longest-shot horse. If your horse wins, save, if not, reload your old save. In a couple of runs at this you'll have enough cash to clear the rest of the game easily.

There aren't really more ways to earn lots of money as fast as possible. The most effective is the horse race technique.

For users which don't know this trick:

1. Go to a horse race bet machine.
2. Save the game.
3. Place the highest possible amount as a bet on a horse.
4. If your horse wins, save. If not load the previous savegame and try again.

The other ways are as you said to do missions or murder drugdealers. They usually have something like 2000$ in their pocket.

Without cheating there's no other "very fast" ways to earn money.

Other optional ways to get money are:

1. Ambulance Job - Bringing people to the hospital.
2. Taxi Job - Bringing people to places they chose.
3. Killing prostitutes.
4. Get a Pimp Mobile and transport girls to and from their work zones.
5. Trucking Job - Do what truckers do.
6. Firefighter Job - 5 Firefighter missions give you like 10.000$.
7. Casino - Black Jack, etc.
8. Quarry - Clear ruble and they pay you.

Feel free to add more if I forget options to earn money.

Roboi’s Food Mart Courier Mission is the first thing I do when I start a new game.

It’ll get you around $10,000 each time you do it, and up to $2,000 in passive income once you’ve completed the mission once.