Collect pre-war books and sell them to the scribe in the Arlington Library. Another great way is to complete the quest 'Reilly's Rangers' and collect map data for Reilly. She will give you a mapping module that 'collects' information on all the locations you visit. When you return to her, she downloads the information and gives you caps in return. You can do this throughout the game after completing her quest. I collected several locations before I went to see her and she gave me 2000+ caps.