My brother had a great trick. The backup light switch was just under the boot of his stick shift rather than down in the transmission, so when someone tailgated him he "blinked" his backup lights at them. It was funny watching tailgaters lock up their brakes to try to not be hit by the transmission they expected to jump out of my brother's car.
I did the same with my Volvo Wagen... the light came on just moving the shifter over, but not engaging it IIRC. The other antitailgater tactic in that car was turning the rear window washer fluid nozzle straight up... I would douche their car and there was a smile whenever their windshield wipers would start. I often wish there was a separate bottle for that nozzle. I would have filled it with WD-40 or waste oil... or battery acid... or piss... whatever.
I heard of one guy who carried a 2M candlepower deerspotting lamp to blind tailgaters highbeaming him at night.