A brother of mine gave me an advice on this recently, goes like this: “just say no! After that find a reason why and give it, but first a no without thinking about it”
I liked it
I'm not arguing against saying "no" to things, but a policy of blanket "no" will leave you pretty unfulfilled.
I also agree that things get done by saying "yes". But, there are only 24 hrs in a day, and if you want to accomplish great things you need to focus, that requires saying no a lot.
To quote Warren Buffet : "The difference between successful people and really successful people is that really successful people say no to almost everything."
This makes people aware your time is limited, makes them do at least a little bit before they get (or don't get) your time and gives you time to decide with a little bit distance.