I can confirm that both JoeyBike and ItsJustMe idea's work extremely well. I also experimented one degree more by towing an empty Burley kid trailer. Cars would occasionally honk at me and drive wayyyy over to the left. Those drivers seemed upset that they were passing with extreme caution, in the same way that they would pass a drunk driver on a busy freeway with extreme caution. When JoeyBike said cars will literally drive into oncoming traffic and force those cars into the ditch just to give you plenty of space he was not exaggerating. This made me a bit sad because most cars passed me safely, it was just the few careless/dangerous drivers that I wanted to force a change in behavior.
I currently use ItsJustMe's idea, but I use one of those plastic orange ChuckIt tennis ball throwers for dogs. I mount it horizontally on my rear rack and it sticks out about 1 inch past my left hand (if you were to sight a line while standing behind my bicycle). I plan to replace it with the orange triangle safely flag that came with my Burley kid trailer. The flag has a fiberglass pole and is pointy at its tip and the pole is very bouncy. I plan to have it stick out 3 inches past my left hand and the flag will bounce like crazy as I pedal. I expect that the careless minivan drivers on their cell phones won't dare risk scratching their paint, as they definitely don't care about passing me in a safe manner.
One thought - if a car hit you and you had a child seat or kid trailer, what would the reaction of the police officer be when he sees your damaged bike laying on the side of the road?