This isn't exactly a glitch, it's just an extremely easy way to make money.
Get $1,000,000, and go to the Four Dragons Casino in Las Venturas. Go make your way to the tables, and look for a big, square table, and wait for it to say "Play Roulette! Max Wager: $1,000,000." If you've never used the casino before, then play some video poker (use the machines that have low valued chips so you don't lose much money) to get your begging level up.
Anyways, once you play, choose either Black, Red, Odd, or Even numbers. Therefore, you have a 1/2 chance of winning, (which is pretty nice.) and once you win, go out and save, then repeat.