Just say no was a tagline from my youth. As I recall, it didn't work well then, but I hope it will work better for a different audience and in a different context today. I was reading Brent Ozar's note about one of the toughest things for consultants to do: saying no. While I certainly think this isn't a problem for some people that can't find work, I do think this is an issue, and one that isn't just a problem for consultants.
I have worked with many people across my career, and one of the things that most of them have struggled with is saying no. Maybe it's fear over being scolded (or worse) by a manager. Maybe it's the simple desire to please others. Perhaps it's because we simple are eternal optimists, and we think we can handle everything thrown at us. No matter what your reason, many people find the inability to say no plagues them throughout their careers.
I've learned to say no across the last decade. I probably say no to more things than I agree to accomplish. There are times that I worry about how much I decline to tackle things at work. However I have learned to deliver what I agree to, and deliver it well. I've learned that to do that, I can't tackle every project, idea, or assignment. There are times I have to push back and refuse (politely) to do something.
It's a luxury for sure. I have been successful in my career, and I'm not in a position where the loss of my job would wreck my life. However I also realize time is the most valuable resource I have at work. Time is also the most valuable resource I have in life, and I have to learn to balance my use of that resource to continue to be successful. Hopefully you learn that as well.