If you spend a few years studying people, you can learn a lot. How the act, how they respond. What made Mitnick so successful? When you see a Con Artist on the street playing a game of cards; or you're at a carnival and the workers there run the games "shoot a basket, win a prize!" Most people pretty much know it's a scam; some will take the bait and lose cash... but Mitnick was different. He used his intelligence and wits to outsmart other people. One of the most effective tactics was to ask for help, he was successful because he acquired the information slowly, just a piece here a piece there. Nothing that would seem at all valuable. You don't ask for a bank account number, but say you need to open a savings account, and ask a recommendation from someone of what bank they use. He didn't offer anything, nor demand it, he asked seemingly innocent, harmless questions which often times lead to more than a wealth of information. The greatest SE comes back without there victims evening knowing they'd been had. The guy wanted assistance with a problem, not to cause one... And most people generally feel good after helping others... Which is why people like Mitnick are regarded so negatively, they play on the trust and good will of others.
Humility and make the other feel important (more than you) always is important (although different persons = different methods, but in general). Seems that Mitnick had the appearance of someone faint-hearted, inoffensive, and those things make the others overconfidents, if the other is "weaker" than you, you control the situation.. seemingly of course.
Also know and get as much information about the other as you can, like Vicarious said is that art of ask the things without do it directly, play with his dreams, desires, trust, requires patience but undoubtedly works.
Based on what I have found out about this guy, he was good, but IMO, he took too much time. If you buy a guy a few drinks, its like saying, "Hey man, its cool, you can tell me everything :)" (oh yea, a smiley in a quote). Speeds up the process tremendously.
He was at good place in good time. But he was catched so he can not be so sucesful. I think there is so many better social engineers but they are not so well-known because they are sucesfull ;-) Hawk
The key to social engineering is patience and the ability to think on your feet. (depending on how/when/where you're doing it).
You've got to be able to size people up in a matter of seconds at times and find out what approach you're going to come at them from.
One of the best ways to do that is to start by appealing to most peoples nature to want to help someone. I think that's one of the reasons Mitnick was so successful, and yes, he was successful. It doesn't matter that he was caught, imprisoned and eventually released. What matters is that he did what he wanted to and now he's made a VERY successful career out of it. Just look at his books and business.
The scope is the incentive. The big question is if Mitnick did it to become famous or did it to become rich. But maybe he did it just because he thought humans are always crowds following similar needs and having similar fears and he wanted to see how far this all common attributes can be exploited.
After reading Kevin's books I think that it was the way he got the infomation. He will find a target and get to know it, spending time learning the languish of the company. It does not matter if it took him a week or a year, just stay on the target until you got it.