I am looking for a way to have the Pi act as a USB client and masquerade as a flash drive.

I have seen lots of suggestions, however most of them are old (1+ years) by now and vague/incomplete - I would like to ask for a refresh as to what worked, what didn't, etc. 

There are also numerous suggestions to use the Pi as a NAS type device instead - this will not work for my purposes, since I have a "dumb" host that can only accept flash drives (and I want to use the Pi to make it "smarter")

summary of suggestions that I have seen already:
1. emulate a USB client via the GPIO pins
2. emulate a serial connection via GPIO, then use existing cables to go from serial to USB
3. using a model A, use software to convince the pito act like a client

could anyone explain more in-depth some of those proposed solutions?

P.S. - I do not need nor expect high performance out of this configuration, USB 1.1 speeds would be sufficient