The way I deal with extreme tailgaters depends on the situation. A simple rule for all situations is to leave a large gap in front so you have some escape room - this saved me and the GF in South Carolina one time with a MACK truck tailgated us and locked up trying to brake hard - I had room to escape into and avoid being crushed by sawn logs.
For 2 lane roads the real problem is overtaking. Modern cars are for the most part crap at it. If you are close to the car in front but have no intention of passing then leave a gap for those in a hurry to pass and pull in to. Then ease off and leave another gap for the next one, rinse and repeat. Give them a cheery wave as they pass, you will laugh at the looks you get. You are not competing against them, you have a smarter opponent - physics.
For roads with more than 2 lanes (i.e. ones with a passing lane) then lane discipline is the key. Move to the passenger side (left in the UK, or the wrong side in the rest of the world
) when you can so that you don't take up more than one lane.
The worst issue on those roads are members of CLOC (Centre Lane Owners Club) - morons who think cruising in the middle lane is the way to go. If you do this you tie up two lanes, yours and the one next to you. That frustrates a lot of other folks who have to cross 4 lanes to pass you.
This is especially true in countries where some vehicles are not allowed into the outside lane line coaches and trucks in the UK. They can't pass anyone dawdling in the middle lane as they are usually not allowed to pass on the passenger side.
If you are still tailgated then the final move is to slow down. In an auto just lift off or slip a manual out of gear. Once the moron behind is doing 30 in a 70 zone he may get the idea.
Brake testing is just stupid for reasons Old Mech has explained superbly. And although a few people around here like it I think drafting is too stupid to use - you lose that escape space in front of you, the space you control and have exclusive access to. Contraversial I know but...