A recent Reddit thread asked, "What are psychological tricks that are useful against people??" The replies provide so many helpful tips for everything from dealing with difficult people, to convincing someone to do you a favor.
If you need a big favor from someone (or a favor someone is likely to say no to) ask a smaller favor first, like borrowing a pen or something similar. They will more likely say yes to a bigger favor.
4 seconds of silence can feel the same as rejection in the brain. So if someone is being a jerk to you, after they say something just look at them and count to 4 in your head, most of the time they'll start backtracking and taking back what they said before you're done counting.