Unfortunately not much can be done. We can be more vigilant and I am sure that now if an airplane is hijacked it will just automatically be shot down or something else to prevent it from flying into anything. Being more vigilant I am sure has helped thwart some attacks but terrorists are determined and they are not afraid to die in order to kill many. But we can learn a lot from 9/11 like people who went back upstairs or didn't leave the South Tower when the North Tower was hit. I think people have learned that the minute something isn't right anywhere near where they are, then run. Work can wait, valuables can be replaced but your life cannot. Other than that though there isn't much else we can do, we still have to live our lives and law enforcement/airport security can only do so much.