Sorry if this has already been posted I can't be bothered to read 4 pages.
Overview:
You have got to remember that these people arn't good drivers. The best place to sit to overtake someone is not on their bumper. It is best to have some room to accelerate into. Therefore they are either not aware they are tailgating (often the case), or they are trying to make you go faster/overtake.
Motorway:
I have found that people often tailgate without knowing that they are doing it. For example if there are several cars ahead on the motorway in the right hand lane of the motorway and the one behind is tailgating you. Now obviously they can't overtake, so tailgating me is pointless as I can't go any faster as I am held up by the people in front aswell. Usually in this situation I want to go faster aswell. So I simply press my crash/emergency light, and leave it on for a few seconds and give them a few flashes. It is suprising how well this works, and they will often drop back. I have sometimes had people put their hands up to apologise.
Sometimes after this when the road becomes clear I am a bit childish and boot it to show him that it was not me causing the hold up, I infact wanted to go faster too. This usually prevents them from tailgating me again.
As a side point I also use this technique to thank people if they let me out of a road. Sometimes when I let someone out I see peolpe concentrating so much on thanking me with hand gestures that they don't really think about the manoeuvre. Madness!
If a car is very aggresive I change lanes, let him past and then pull back out behind him. This shows him that I was not dawdling and gives him the opportunity to force the people infront out the way if he so wishes. I leave a suitable distance and am clear of the problem. Usually I find that when I do this they tend to not tailgate the person in front and get the message.
Single carriageway:
In traffic I use the same technique as above. If the road is clear and they genuinly want to go faster than me I will slow down to make it easier for them to overtake in suitable areas i.e. not on a corner where they can't overtake anyway (this will infact annoy them more) but definitely on a straight. You need to exagerate this if the road has less overtaking spots.
If it a very small road, then I guess consider pulling over (I have never had to do this).
You should not do the following:
Slowing down on purpose to prove a point is a very bad thing to do. Presumably the reason that the person is tailgating you is because they wan't you to go faster. As for brake testing well this is very childish and would just annoy the driver more and lead him to just be more aggressive
Conclusion:
I find that people normally don't want to travel faster than I am going (except on a motorway but this is not a problem as if the road is clear enough to go faster than I do, I will not be in the outside lane )

. So the only time I am tailgated is if there are cars infront when my crash light method usually does the trick.
P.S If I am on the motorway and I feel the person infront should let me past e.g. sitting in the right hand lane with no traffic in front, then I hang back and indicate right. I found out this is what the Germans do on the autobahn and it is considered the only polite way to ask someone to move over. This also seems to work and people appreciate the fact that I havn't tailgated them.
Wow that was longer then I expected!