One of the things I struggled with right when I entered "adult life" was saying no. When opportunities came up at work, or someone needed me to do something for them I would always say yes. In moderation though, that's fine to an extent. If a coworker of mine asked me to get lunch with him when I should be networking and not catching up, that's when I should typically say no.
Across the board, no's are powerful. Whether they have to do with your career, friends, or even family.
I believe the most powerful no in history came from God. In John chapter 12, Jesus was predicting his death and one of the things he was contemplating was this: "Now my soul is deeply troubled. Should I pray. 'Father, save me from this hour'? But this is the very reason I came! Father, bring glory to your name." (v. 27-28)
Jesus seemed to be at a point where he was thinking to himself...is this all worth it?
Briefly after Jesus said this, a voice from heaven spoke and said: "I have already brought glory to my name, and I will do so again." (v. 28)
That is the most powerful and important no in history. Because if God said: sure, I will save you from this hour..." we wouldn't be here and Jesus wouldn't have died on the cross for all our sins.
God just always seems to work in the most unique ways...I was listening to a podcast on Sunday when I was on my way to pick up my wife from the airport and a lot of what I just wrote about was talked about in that sermon. For the life of me I couldn't figured out where that passage was in the Bible, and my daily reading for today were those exact verses.
I don't know if God is using this because I need to get better at saying no, or because someone who is reading this right now needs to. My encouragement to you, is that it's okay to say no. 99% of the time when you say no to somebody, they're going to give you grace and understand. They might jokingly give you a hard time, but they're going to be okay with it. For the small percentage of people who are upset with you, maybe it's a good reason you said no and shouldn't be associating with people like that. Please don't put too much on your plate for saying yes all the time.