Actively worshipping the god of your choice may be your only chance. You know, we need a saint of cycling. Who's the saint of lost causes again?
I have a fabric basket on the left side of my backrack. On it I put an obnoxious safety triangle, front and back. It gives the impression of that being the center of the bike, so people give me more room. They'll practically run head-on into oncoming traffic just to give me six feet, that's all I know. They didn't budge for me before.
I like the idea of the stick with the flag off the left side--I've heard of people using them, but I don't know where they got them. I did read a comment once by a tourer who said teenagers like to see how close they can come to the flag with out hitting it. They found they were harrassed far more often with it. In fact, they had it clipped off by a driver once while riding with a large group. They still use it though from what they said.