I’m not sure how much of this isn’t common sense but the espionage information is particularly interesting.
“The basic tips: Don’t bring too many new items in your luggage (sorry, Christmas gifts); don’t be nervous and visibly sweating; don’t change your itinerary within a day of your flight; don’t be weird about passport screening procedures; and make sure your luggage isn’t disproportionate to your purported reasons for your trip, which could mean how it’s packed or how fancy the stuff on the inside is.
The documents also contains security procedures from around the globe, shining a light on what other countries look for in a passenger profile. Turkish accents in Iraq get you an almost automatic screening. In Israel, lone male travelers with backpacks often get secondary screening, while lone travelers from China elicit concerns of illegal immigration from Chilean authorities.”
To start, don’t be a sweaty with an Apple store in your luggage, after that it seems much might not be up to what you can control.
CIA Leak Reveals Who Gets Pulled Aside for Extra Airport Screening (Popular Mechanics)