Sometimes, saying yes can be a boon to your career—it opens you up to new challenges and opportunities, invites collaboration, empowers and affirms others, and creates an environment where it’s safe to try, fail, learn and innovate.

But always saying yes can also leave you exhausted, stressed, and time-poor, wondering why you’ve been busy but not productive.

Billionaire Warren Buffett, the chairman and CEO of Berkshire Hathaway, has a theory on this subject worth exploring: “The difference between successful people and really successful people,” he says, “is that really successful people say no to almost everything.”

Notice that Buffett said almost everything. He is speaking to our ability to master decision-making and time management. He says we must choose, with intention, what we say yes to and what we say no to. It all comes down to simplifying, prioritizing, and focusing our attention on what matters most.

Tech wizard Steve Jobs agreed. “People think focus means saying yes to the thing you’ve got to focus on. But that’s not what it means at all. It means saying no to the hundred other good ideas that there are. You have to pick carefully.”

If you want to grow your career, you need to learn to say no. Here’s how.

Be honest about what’s truly urgent and important.

It’s easy to get overwhelmed by the sheer volume of to-dos and requests, believing that everything is urgent and important, which seldom is the case. Instead, realize that not all tasks are created equal.

Utilize a simple decision-making tool like the Eisenhower Box, in which you place to-dos in four boxes: Do it now for urgent and important items, Decide to schedule a time to do it for non-urgent but important tasks, Delegate it to someone else for urgent but unimportant things, and Delete items that are neither urgent nor important.

Successful people say no to everything except what’s both urgent and important.

Treat your time like the valuable and precious commodity it is.

Time is a limited and non-renewable resource. It’s hard to be productive and focused when you have many things vying for your attention. This time of year, especially, we feel obligated to say “yes” to every request, but those tasks can distract us from what we really need to do and can lead to burnout. And you can’t accomplish your goals if you’re overwhelmed and overworked.

Successful people recognize that if they can’t take care of themselves, everything else suffers, and this starts with managing their time. As Buffett affirms, “You’ve gotta keep control of your time, and you can’t unless you say no. You can’t let people set your agenda in life.”

Ask yourself if this will help or hamper your goals.

Being agreeable and maintaining an open mindset is great for collaboration and adaptability, but can eat away at your available hours. Become more discerning with your time by asking yourself two questions: What do I want? Will saying yes to this (request, event, activity, task, etc.) help me achieve that or prevent my progress? If it doesn’t excite you, speak to your values, or further your mission in life, say no.

And this doesn’t just apply to things; consider with whom you’re spending your time. If your environment, which includes the people around you, is filled with negative, overly critical, or uninspiring people, then it’s holding you back from accomplishing what you want to do. It’s time to establish firmer boundaries and only allow those who inspire, support, and challenge you to reach for the stars.

Remember, when you say no to the things and people who no longer serve you, you can say yes to those that do.

This article has been reprinted with permission from Amy Blaschka’s Forbes page.