I always speed, so when somebody tailgates me its even worse. When I catch somebody tailgating me I will drop my speed down to the limit just to piss them off. On most occasions they get the hint and back off.
The best thing to do is when you're tailgating someone else.
Like he's going 20 in a 35 for some reason, if you can pass them, do it. Hell, do it even if you can't pass them. Honk and make a big ruckus, maybe shout out the window "drive faster you aging twat" and speed off.
Then, slow down to 10. And make them sit behind you going 10.
Then, when they think: "ok that's enough" you drop it to 5.
I did this once. Best thing of my life. I could see the guy's face in my rear view, loving pissed off as hell. Just when I thought he was ready to pull over and start chasing my car, I sped up to about 45 and lost him. I don't think he'll drive slow again.