I slow down gradually, often well below the speed limit. I don't nail the brakes like I do in my car in the same situation. I keep an eye on them to make sure they SEE me slowing down. Usually they get the hint. I've only had one b!tch continue tailgating me and getting MORE aggressive so I let her by. I tried to give her "the look" as she went by but she refused to look.
I don't think I'd flip them off or throw anything at them. When all is said and done they outweigh me by at least five times. Can't argue with that if they want to escalate things.
