There are books on p2p networks on this subject. It's so easy when you know how.
It's mostly about blanket statements and educated guesses. I.e.
"I believe you are an honest person, but you occasionally tell white lies to protect people".
There aren't many people who would disagree with this as most people like to think of themselves as honest and most people feel they can justify their lies.
"I sense that you have been heartbroken in the past and find it hard to fully trust someone".
Most people have been heartbroken at some point in their lives, and if the person is single, they have not met the partner who has earned their trust, so it is a safe bet they don't trust anyone fully.
Also, you can judge a lot from the way a person dresses. If she is dressed colourfully or unconventionally, you can say:
"You are a creative person who likes to express yourself. You want to be heard and want people to notice your ideas....and so on."
If she's dressed classy, you adapt and say:
"You are very driven and have your sights set on success...and so on".
It's all generic stuff, just like all horroscopes, tarot and palm readings etc. Most people fail at this by trying too hard and trying to be too personal.