>What can we do to stop things like this from happening again?
The best way? Kill this stupid mentality of "what can we do to make sure this never happens again". It is exactly this mentality that lead to his death: 911 happened so we started a war against a concept and started invading people's countries, killing good people from both sides.
Norway is the example we should all follow: when something terrible happens, morn and punish those directly responsible but don't change anything about yourself and don't go off with lunacy like fighting ideals.