The distraction principle. While you are distracted by what retains your interest, hustlers can do anything to you and you won't notice.The paper also discusses a dozen con scenarios, which are both informative (and entertaining).
The social compliance principle. Society trains people not to question authority. Hustlers exploit this "suspension of suspiciousness" to make you do what they want.
The herd principle. Even suspicious marks will let their guard down when everyone next to them appears to share the same risks. Safety in numbers? Not if they're all conspiring against you.
The dishonesty principle. Anything illegal you do will be used against you by the fraudster, making it harder for you to seek help once you realize you've been had.
The deception principle. Thing and people are not what they seem. Hustlers know how to manipulate you to make you believe that they are.
The need and greed principle. Your needs and desires make you vulnerable. Once hustlers know what you really want, they can easily manipulate you.
The full paper can be found here (PDF format): "Understanding scam victims: Seven principles for systems security".
