In order for this to work,you need Megaton city to be in one piece and have a fair sneak ability.

1) Enter Megaton and go to Crater Supplies. Talk with the funky Canadian lady and trade with her so she ends up with the majority of the caps (as you progress through the game and buy from her, the amount of caps at her disposal will increase).

When you're done, notice the desk on the right as you come in. Push her slowly behind it(sometimes she's a pain to get behind there), but push her so she is at least close to the desk and you can get behind the desk and crouch, and the status says "hidden". You should be just close enough to pickpocket her.

Her inventory should list the majority, if not all the caps she had when trading. Once you steal the caps ,she will notice, so run as fast as possible out the door and immediately pull up the pipboy and fast travel out of there.

If you did it fast enough, there will be just enough time for you to fast travel out of there before the people turn hostile. Now, you will be able to fast travel back to Megaton after a couple of minutes, if not immediately, and everyone will be calm again.

Then ,repeat the process!

Note: this DOES cost you karma points, but it's well worth it for the few points of karma you sacrifice, as now you can stock up on stimpacks.

Try it, it works... and she always has money.