Go to the town of Primm, and successfully complete the "My Kind Of Town" quest to get them a new sheriff. Wait 3 to 7 in game days for the Vikki and Vance Casino to open again in Primm. Then, go to the Vikki and Vance Casino, and either win as many chips as possible from gambling or exchange as many caps for chips as possible. Go to the cash register to trade the chips in for caps. You will get the equivalent for the chips in caps, but you will still retain all your chips. Simply keep trading in the chips for caps to get as many caps as desired. Note: This glitch was done on an unpatched version of the game.
Go to a weapons and/or armor merchant, and see if he has two of the same weapon or armor for sale. If you can afford it, get the overall most expensive item at the lowest condition. For instance, if he has two Raider Sadist Armors at 45% and 76$, but also has two Flamers at 20% and 8%, then choose the Flamers. Purchase the cheapest of the two items, and then immediately sell it back to the merchant. Buy it again, and sell it back to them again. Usually the second, but sometimes the third time, you buy the same item from them, and it will appear in your inventory at 100% condition. Sell it back to them, and it will reappear in their inventory at the original low % condition. Buy it again, and it will appear at 100% condition. Repeat this until they have no more caps. If you wait three days after they run out of caps, they will have new stock and approximately 650 caps. The longer you wait, the more robust their inventory will be and the more caps they will have. Note: This glitch can cause the game to freeze. It is recommended you save your game often when doing this just in case the game freezes.
Successfully complete the "Wang Dang Automatic Tango" quest to find Fisto, a Sexbot. Then, go to Silver Rush Casino, and talk with Fisto next to the roulette table. He will offer his services. Choose the "No thanks" response to get 10 caps. You can keep selecting this answer as many times as desired to get 10 caps each time.