1. If someone doesn't stop talking or let you get a word in, drop something (keys, pen). Reach down to pick them up and start talking. It's a way to interrupt without the other person realizing it.
2. When something funny happens and people or a person in a group laughs, they will look at the person they like or care about. When you notice this it's quite easy to tell who likes who.
3. If you want to calm someone down, sympathize with them whilst describing what's upsetting them in descending orders of magnitude. I understand why you're angry. You're right to be frustrated. This would annoy me too. This is what you're doing. You're showing empathy, you acknowledge their feelings, and don't start with giving advice.
4. If someone is talking or preoccupied, you can hold out your hand, and they'll give you whatever they're holding. I did it to my brother with the car keys and made him think he lost them somewhere at the store.
5. My favorite is silently maintaining eye contact when a person is attempting to bargain or convince you of something that you don't want to do, or don't believe. They usually end up negotiating with themselves (which gives you a huge advantage because once that happens, it is pretty much game over).
6. If you work with clients, prevent them from getting defensive and angry by not using the word "you". It's not "you didn't send the attachment", it's "the attachment didn't send". Don't blame them, blame the thing you're talking about.

One word can be the difference between a happy client and an unhappy one. The right words in the right order can influence people to make decisions you want them to make. Words are powerful.

Source: What are the coolest psychology tricks that you know or have used?