There are answers, but not many that are politically acceptable in the current environment.

Something very close to "profiling" is going to be required. The prime suspects are Middle Eastern men. Deal with it. Start tightening the borders. Start checking out locals--particularly non-citizens--within the bounds of normal police procedure. Very politically incorrect, but it's time to tell the leftist looneys (who are less interested in freedom and individual rights than in just kicking America in the shins) to shut up and sit down.

I don't think we have to try for ultimate safety at the expense of all our citizens' freedoms. Just keep the risk as low as possible within the bounds of what we can do as a free nation.

Keeping an eye open is not a bad idea, despite the jokes made about it. A possible massacre at a local college here was nipped in the bud because a girl operating a 1-hour photo machine saw some pictures go by of a kid posing with heavy duty weapons and called the cops. She could very easily have just shrugged and gone to lunch.

But you know what? We're already hunting the terrorists down across the planet. They are having a tougher time of it than they used to. We have the time, the money, and the resources. We'll find them eventually. With luck, we'll find a lot of them before they can do anything. Eventually, we'll find bin Laden and the rest, or their corpses. They need to be lucky all the time, we just need to get lucky once.

They may blow up a building, or a mall. It will be awful, but it won't beat us.