Three Ways Easily!
1. Um, its pretty easy, when you hit about lv14 (I think), you get this Perk wich make the bad guys drop fingers. Those could be traded for 10caps each, and I'm guessing they're about 20% drop rate, its a good way of making some caps while levling. :) [GOOD KARMA]
2. There is also Paradise falls, the slave shop town. When you started a quest called "Getting into Paradise falls", you will have to run there and talk to the guy outside. After you've talked and made a deal with him about capturing "VIP" Slaves. He will give you a weapon (Met-something)and you will have to capture some slaves. He will trade each slave for 250caps each. Please notice that you will get Bad Karma. [BAD KARMA]
3. The last way (atleast of what I use) is to trade Pre-War books with the Arlington libery receptionist lady. She will give you 100caps per Pre-War Book, wich is a pretty fair deal. [Neutral / Good Karma]
I hope this will help, enjoy. // BathJelly @ Xboxlive.