Most of the above suggestions deal with the passengers on an aircraft, and rightly so.
But a point we may have missed is, what about the ground service employees at an airport, namely the aircraft cleaners, fuelers, ground handlers--shouldn't they also be screened with background checks upon hiring? My point is this: How did the terrorists get their weapons aboard the planes? Were the metal detectors and scanners ALL so inefficently used that all of the knives were missed? Isn't it just possible that the weapons were already on the aircraft, hidden by associates earlier, even before the terrorists got on the planes? Just speculation, of course, and we may never know the answers, but a possibility.
I think this possibility should not be overlooked, and stronger security checks for ground staff should also be put into place.
Just my 2 cents.

bj-21.