Ease off the gas and let your vehicle slow down gradually. The tailgater WANTS you to go faster. Do the opposite of what he wants.

I had this happen to me on US 54 through Texas. It was early in the morning so still dark, and I could see this guy hanging off my back bumper. If it hadn’t been dark I might not have seen him but the light from his headlights kept peeking out and hitting me in the eyes. So I started slowing down…and I kept slowing down…and I kept slowing down…

I literally got down to 20mph on a 65mph road before he finally took the hint and passed around me.

EDIT: Since a few people don't seem to be getting it, I drive a semi truck. It says “truck driver" right next to my name, but apparently some people don't bother to pay attention.