The wealthy and successful share a common trait: They say no -- a lot. They say no to annoying requests to pick their brain, to lukewarm opportunities, and to anything (or person) they deem will be a waste of time.

The benefits of saying no are simple enough. It frees us up to focus on projects, people, and passions that really matter to us. There is, of course, a complication. It's not always easy to decide what to say no to.

Tackling decision fatigue

Should you attend that networking event? Should you volunteer your talents for that optional-but-not-mandatory thing at work? Take that job offer you're lukewarm on?

These decisions can be hard ones to make because there's no clear right or wrong choice. Saying yes might also present some benefit. You might meet someone at the networking event who can help your career. That work project might get you more visibility with the higher-ups.

And yet, there are cons to saying no.

These are situations where you can employ the 90 percent rule. Coined by author Greg McKeown in his book Essentialism, this fast and efficient decision-making framework was recently revisited in The New York Times'  Smarter Living section.

Use the 90 percent rule to make guilt-free decisions

Start with 100 imaginary points. Looking at everything else you have on your plate and what you have time for, how excited are you to do this? How many points would you give it if you were to move forward?

If it's above a 90, McKeown says you should go for it. Anything below, it should be a hard pass. A 50-50 is just as well a zero. If you've given a task a 75 percent score, that's also a zero. (If it's easier, use a 1-to-10 scale. The same rule applies. Anything below a nine ultimately is the same as a one.)

So say no. Feel free to move on, because you made the right choice.

When you know that no is the right decision

This may seem harsh, but McKeown doesn't think so. We have only so much time to give every day, every week, and every year. In the end, you'll get more done and live a richer life. You'll have less to do -- and more time to do the things you actually want to do. 

Time-management expert Laura Vanderkam also heralds this technique to prioritize your time. She says if more people used the 90 percent rule to decide how to spend their time, there would be fewer crazed, harried, stressed-out workers among us. "We don't build the lives we want by saving time," Vanderkam said in her TED Talk. "We build the lives we want, and then time saves itself."