TomK said:
It always makes me slightly cringe when attempting the same as I am well aware there may be a total knob head who doesn't look in the rear view before going for the overtake.
One technique you can use is when you pop out for an overtake in a line of cars and it looks a bit dodgy (someone else looking for an overtake for example), hold your full beams on for a few seconds. Yeah some people might not get what you're doing, but if it catches their eye in the mirror that's all you need to achieve. Sadly some people will probably see it as antagonistic but, that's their problem. Though best to use it only when feel necessary rather than routinely!