In order to do this cheat you have to complete the My Kind Of Town quest.

After that discover the Strip, then go into either the Gomorrah or Tops Casino and play one round of any game.

Then head back to the Primm Vicky and Vance Casino. Go to the clerk and exchange caps for chips. Turn in the chips.

You will notice that your chips don't go away! Keep doing this over and over again. The more chips turned in, the more chips gained.