ChristiaanJ,
I did mention the dye marker previously, but it seems so obvious that I would not claim to be first with the suggestion.
I have seen dye markers used in a marine trial, and the range from which they could be seen was most impressive.
It is true that the dye will disperse rapidly, but it needs only to be present for long enough to be seen by the first search aircraft, which can record the GPS coordinates.
I cannot guarantee it would be 100% reliable, but it would be very simple, cheap, and not require any modification to existing aircraft systems. None of the other suggestions put forward meets these criteria.
Putting dye markers on lifejackets might help with finding survivors, but would be misleading when looking for the sunken airframe and FDR because floating debris is scattered widely by wind and waves.