Deciding what to pursue --and, just as important, what not to pursue --is one of the toughest choices you can make.

That's whySteve Jobs believed in the power of saying no, so much so thatJobs askedthen Apple head of design Jony Ivethe same question just about every day:"How many times did you say notoday?"To Jobs, saying nocreated focus. He didn't see being "busy" as a proxy for output, or seriousness, or productivity.

That's why, when asked to describe what sets successful people apart,Warren Buffett said,"The difference between successful people andreally successful peopleis that really successful people say no to almost everything."

Problem is, sometimes you do need to say yes. So how can you decide when an idea or goal is truly worth pursuing?

Ask yourself, "Is this trombone oil?"

In The Ride of a Lifetime, Bob Iger's excellent book about his 20-year career as Disney CEO, Igerdescribes a lesson he learned from his longtime bossDan Burke:

I can't recall exactly what it was in response to, but in one of our conversations about some initiative I was considering, Dan handed me a note that read: "Avoid getting into the business of manufacturing trombone oil. You may become the greatest trombone-oil manufacturer in the world, but in the end, the world only consumers a few quarts of trombone oil a year."

He was telling me not to invest in projects that would sap the resources of my company and me and not give much back.

I still have that piece of paper in my desk, occasionally pulling it out when I talk to Disney executives about what projects to pursue and where to put their energy.

Before you say yes to an idea, before you decide to put time and energy and resources into the pursuit of a goal, make sure the return is more than worth it. That return may be financial. Or philanthropic. Or personal.

After all, you get to define "success."

But since your effort, energy, and resources are finite -- since any time and moneyyou put into one projectcan't be put into another project -- make sure the outcome will more than matchinput.

Because if it doesn't,you'remanufacturing trombone oil.

The opinions expressed here by Inc.com columnists are their own, not those of Inc.com.

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It Took Disney's Bob Iger Only 1 Sentence to Give the Best Advice You'll Hear Today - Inc.