Just make sure you can carry a lot of loot back to sellers. The guys at Rivet City (Flak and Shrapnel) usually have a decent amount of caps. Avoid items that have a lot of weight for their cap value (unless they're needed to make custom weapons).
Make sure you repair most of the stuff you're carrying. That saves some weight.
And get an NPC companion. Charon is one of my favorites, though Fawkes can carry way much more weight. She (yes... she!) appears quite late game, though.
I made one of each custom weapon and STILL have not earned the trophy. Man, how annoying!
Make sure the shadow you chase is not the one you cast.