If there's a big problem in corporate America, it's that we say "Yes" too much at times.

Yes to that request..

Yes, I can help you..

Yes, I'd be happy to be part of your project team...

Yes, your response to my request is fine...

There's a whole lot of yes going around.  The problem?  Only about 1/2 of the "yes" responses are followed up with action that is representative of all of us living up to the commitment we made.

That's why you need to say "no" more.

Of course, simply saying no with nothing behind the no positions you as jerk.  So the "no" has to have qualifiers behind it:

Say "no" more to peers asking you for things, but then qualify it with how the request could be modified to move you to say "yes".

Say "no" more to your boss, and qualify your response to her by asking for help de-prioritizing things on your plate - which might allow you to say "yes" to the new request.

We say "yes" in the workplace when we want to say "no". We do it because we don't like to say no, and because we are horrible at negotiation.

Say "no" and tell people how the request could be modified to get to "yes".

Or just say "no" and walk away.  Either way, you've helped the organization's overall performance by providing more clarity.