Psychology major, here!

If you want someone to say yes to something you're asking for, here are some options:

Nod as you ask. Our brains have what are called 'mirror neurons' which activate to make us mimic other people's actions. Sometimes they will also start to nod, which then makes them then agree with you. I learned the mirror neurons part in psych class, then the nodding part when I worked in sales. Shit's legit.

If you have a big favour to ask, start by asking smaller ones ahead of time. This is called the foot in the door phenomenon, I believe. If you ask for small things, then work up to big things, people are more likely to say yes than if you start off with a huge favour.

Inversely, if you are asking for a small favour but still know that they will probably say no, start by asking them for a huge favour. Then when they say no, ask for the smaller one. They're more likely to say yes, especially if they feel a bit bad about turning down the first request.