Are there any tricks / methods / accessories that can be used to mount an off camera flash unit on a tripod?
I got a Frio as a freebie in a photography class grab bag. Its a nice option: it mounts directly to the tripod mount stud, and since its all plastic, it doesn't harm your flash contacts at all. It is about the size of a thumbdrive, so it fits in your pocket. Its overpriced as a piece of plastic, but quite affordable compared to other more complex options.
You need something like this:
Or maybe this one, for a lot less money:
There are lots of other models. Some, like the top one shown here, provide a hot shoe so that you can trigger the flash using a cable with a PC connector or, in some cases, a mini-plug. Others, like second one shown here, only give you a cold shoe, i.e. a way to hold the flash but no way to trigger it. You'd use when you're using the flash as an optical slave, i.e. triggering it with the light from another flash.
Two other options that I use: An umbrella mount usually has threads for a tripod on the bottom and a cold shoe on top. As a bonus, you can now also shoot with an umbrella if you want. I also use a cheap radio trigger (ctr 301p) to trigger off camera flash. It has a tripod thread on the bottom and a hot shower on top.