I extend my right arm and point towards where I want to go. It seems to me that the left arm bent up is an automotive thing, as a driver sitting on the left side of a car obviuosly can't reach out of the right side window! Pointing, though, is a more intuitively-understood gesture, I think.
I know that the left arm bent up thing is the "approved" method, as taught in driver training, but I wonder how many drivers would remember/recognize this as a right turn signal anyway.