Some say that saying yes is very powerful, while some argue that saying no is more powerful. Unfortunately life doesn't come with a manual to teach you when you should say no and when you should say yes. And knowing the difference of when to apply which, is what changes your life positively.
I think the best way to approach this is what I read from James Altucher. He wrote:
"I started saying “No” to people who weren’t right for me. I started saying “No” to everything I didn’t want to do.I started saying “No” to mindless meetings, mindless events, mindless people who were bad for me, mindless food or alcohol, mindless anger and regret. Mindless TV and news.I started saying “No” to colonoscopies and other things related to painful medical experiments. I listed all the things I could say “No” to and I still do.When you have a tiny tiny piece of shit in the soup it doesn’t matter how much more water you pour in and how many more spices you put on top. There’s shit in the soup.I had been saying YES to the wrong things for 20 years.Within six months my life was completely different. I met Claudia. I moved out of hotels. I was working on ideas that actually made money. And I needed fewer and fewer things to make me happy. That’s the Power of No. That’s true minimalism."