Bruce schneier makes some interesting comments about patterns of behavior and tactics. In particular, the key point is the following: If you have an attacker who has a consistent pattern of not attacking you the same way twice, it's not only ineffective to put in place massive preparations against a repeat of each new one-off attack, it's stupid, because it diverts resources from emergency preparedness measures that you could use to respond to ANY attack or disaster.
Al-Qaida terrorism is different yet again. The goal is to terrorize. It doesn't care about the target, but it doesn't have any pattern of tactic, either. Given that, the best way to spend our counterterrorism dollar is on intelligence, investigation and emergency response. And to refuse to be terrorized.
These measures are effective because they don't assume any particular tactic, and they don't assume any particular target. We should only apply specific countermeasures when the cost-benefit ratio makes sense (reinforcing airplane cockpit doors) or when a specific tactic is repeatedly observed (lions attacking people who don't stare them down). Otherwise, general countermeasures are far more effective a defense.
There's only just so many one-off attacks that even the US can afford to put billion-dollar infrastructures in place to defend against a repeat of. And every time we do, every time we overreact, every time Congress wets its pants and hides in the corner, every time we return an airline flight to the airport because a passenger had hand cream in her carry-on — every time we throw the baby out with the bathwater — we hand the terrorists a victory. The way to win against terrorists is to deliver a swift and deadly beatdown whenever we're sure of the right target to respond against, and otherwise refuse to allow them to change how we live our lives. Deny them the terror.