It starts with knowing your passengers, the company they work for, the totality of the situation....plain ol' good commonsense. No security system is undefeatable....one can only make it harder to succeed in an attack or breach.
Now given a choice of a 757-767 sized aircraft...or a Jetranger...now which one do you think I will go after? If I am deterred from the large aircraft...then there are cargo aircraft...then Business Jets of some size...then...well somewhere down there are the small helicopters....still a target but not as lucrative unless you have a very small but powerful device to deliver.
Do any of you know what a backpack nuke or suitcase nuke looks like?
What does a container of Sarin look like?