If this attack shows anything, it's that terrorists can and will be successful regardless of when, where or how they plan to execute an attack.
This may seem odd, but I honestly think the best course of action would be to have militia like we did back in the 1700s. Everyday Joe's who train occasionally and are armed and, at a moment's notice, will assemble to protect their community.
Not an Army, not a National Guard... private citizens who can come together armed and ready if and when there is an attack.
I don't consider myself an eccentric person, and I don't even have a CC permit. But terrorists prey on the week and defenseless. Since you can't expect everyone to be armed all the time (some simply refuse to own a gun, period) and the police and military are always a few steps behind, a militia is the next most obvious line of defense.
Heck, it might be enough to at least make terrorists think twice. A single man with a truck killed nearly 100 people during a peaceful national holiday celebration. One dude. It wasn't until he'd driven 3 miles that he was taken out by police.
15 years ago, hijackers brought down the World Trade Center buildings and killed nearly 3,000 people in the process with a few commercial airliners.
Think about that.