Honestly, mostly observation. Noticing how people respond in different situations. I used to be good enough to look at a complete stranger in a social setting and predict what they would do next. No longer at that level though, at that point I was so clinical I couldnāt operate socially very well.
The topics which helped me most are things like reframing (used to be a big thing in the days of outer game, taking whatever is said and finding as many possible different interpretations as possible, picking the most useful one), language patterning as used in conversational hypnosis and NLP/DHE and figuring out how to write affirmations and basic subliminal messages.
Practicing reframing on everything is magic in your life. Combine it with reading something like Melvin Helitzerās Comedy Writing Secrets and you can turn anything into anything else.
Mission accomplished. 
Actually, Iām being intentionally transparent here, otherwise it would be difficult to explain. When put bluntly like this, it sounds manipulative in a bad way, but in general I do aim for the win-win scenarios. Even if thereās somebody I donāt like thatās bothering me, giving them what they want somewhere else helps us both while helping me improve my mad skillz.
Thing is, I notice so many people are just getting in their own way, myself included. If I can help them get out of their own way, it feels good. And Iām still hoping somebody would do the same for me. Unfortunately Iām equally adept at recognizing people trying to influence me.
Remarkable makes a good point about turning it into a game. When I observe people to figure out what motivates them, it is a challenge for me, see if I get it right and what I can do with that. Often enough that is enough to stop me from blurting out whatās on my mind. I also tend to take a breath before I say anything (or correct my posture), gives me time to think and usually gives more gravity to my words. Well, the impression of having more gravity anyways.
The curious thing about remarkable is that he has no filter, no spin. He tells it straight, radical honesty (thatās a book also by the way and an interesting way of going through life). Most of us will try and cushion the blow, getting the message across without using pressure points. Not remarkable. I donāt mind personally, but I can often feel the sympathy pain, like when another man gets a kick in the nuts.
