All the so-called technical solutions that I've seen so far have been, frankly, loony. The panic button in the cockpit, or PIN numbers solution, is just totally unworkable.
The only answer I think is in controlling access to aircraft, and this depends on much better surveillance at the boarding point. We could probably assume that any would-be hijacker could be highly aroused while going through security procedures, and it should not be impossible to monitor heart-rates while passengers pass through screening. OK, so a few nervous fliers may get falsely picked out, but maybe it could catch the bad guys as well.
If we plug all the gaps in passenger security, where does that leave cargo aircraft? I have a vested interest here, and I am convinced that it would be easy for terrorists to conceal themselves in a cargo pallet. Once aboard, they would have only two or three crew to overcome, and then be in charge of an (often) very heavy and highly fuelled missile.
Some kind of check of cargo must be instituted, based on the technology now used to find illegal immigrants in lorry cargo.
As for the idea of "Trojan Horse" flight crew, infiltrated into airlines, is this really any more likely than infiltration into Nuclear Power plants, the Military or other vulnerable areas?