We keep a "SORRY" sign in the map holder on the driver's door and the passenger's door. It could also be kept under the sun visor if it is fastened with a clip or rubber band so that it doesn't hit you in the face when the visor comes down.
Warning: Use of common sense is essential in responding to road rage incidents. Quite often, getting out the sign or turning on the light will be a serious distraction from safe driving. If using the "Sorry" sign makes your driving unsafe, do not do it! Practice in using the sign when the car is not moving, such as in your driveway, will help to make use of it safer and easier. Practice will also help you determine whether using the sign in a particular situation will be more dangerous than not doing so.
Seems less complicated than the fixed Thanks Tail, but it would be sad if a clumsy sorry manoeuvre accidentally provoked an incident.
(via bifurcated rivets)