@MichielAndreae This guy was definitely ahead of its time and most of those social engineering skills are still used today.
I have been advised to read the art of human hacking too. I haven't read it yet, but it seems pretty similar to Mitnick's book with a slightly different angle. Take a look at that review of the book.
@vvanedwards I'll try to avoid the ones you already taught many times in your courses though I use most of them naturally.
My favorite social skills would be "trust your instinct/feelings". That may be a Lylatwars (N64) remain where Peppy said "Never give up, trust your instincts !" but I believe this is true most of the time,
The second would be to stay true to yourself. If you have an opinion, you can share it and defend it. You don't have to agree with anyone, you just have to respect their right to have a different opinion. In my experience, you usually come across as intelligent and confident if you do it well.
The last one would be to give everyone a chance. You can learn something from everyone, give them the opportunity to teach you. That's particularly true in hostels, everyone is there to have a fun moment in a foreign city and the "where are you from ?" or the "How do you know each other ?" work every time. I love hostels just for that, you are alone only if you want to, I've never seen anyone be rejected (maybe this has to do with the traveler's mindset...).