I’m still very much in the earlier stages of learning, but I’ll say negotiation. Recognizing that virtually any situation involving an “exchange” between two interested parties can become a negotiation has been a great insight into control for me. People often think of negotiation as business, however it can be much more personal than that and can be very useful for your interpersonal relationships as much as it can…
The unfortunate thing about negotiation books is that people begin to see all forms of human interaction as a negotiation. Negotiating when buying a car, sure go for it. But if you've scratched someone else's car and they're asking you to pay for it. Maybe not the best time to whip out those negotiating skills.
That's a perfect time to use those negotiating skills. When I was buying a car, I just told them the number I'm willing to pay, and two weeks and 5 calls later they agreed. On the other hand, had I scratched someone's car, I'd want to use some negotiation tricks in case they wanted me to pay too much.