If it's a car or SUV tailgating me, I like to turn on my windshield washer. Usually the water spray will end up on his windshield and they are forced to clean their windshield too. They usually fall back a bit. If they come closer, I just repeat myself. Works great, especially on motorcycles and open top convertibles.
Another thing to do is to tap your brake pedal lightly while stepping on the gas. Tap enough so your brake light comes on but you are not actually slowing down. They'd pull back too, very effective method.
Thirdly, you can just take your foot off the accelerator and let your car slow down, annoys the heck out of anyone behind you.
Another method is to turn on your signal, make like you're signalling to get to the slower lane but remain where you are. Damn annoying too.
Of course errant tailgaters will react by pulling closer or flashing their high beams at you. They'd tire easily and be on their way...