I've learned it's better for one's career to never say "no". This includes, "It can't be done" and "I don't have the time/resources". You are perceived as "negative" and the opposite of "can-do" if you say "no".

Rather than "can't be done", say something similar to, "I'll look into that. I don't currently see a way, but will continue to research it." (In some cases mulling it around in one's head over time will indeed help one think of solutions, or satisfactory alternatives.)

If it's about time/resources, then either say, "I have a lot on my plate right now, but will see if I can work it into my schedule." Or, show them your task list and ask what should be moved down in priority to fit their request. This will move the political implications into their court, and also remind them of your work-load. Be polite about it, though.

Yes, you may end up being considered a flake or procrastinator for some of these, but that's better than an up-front "no". It just doesn't go over well.

(And please, no personal digs about not finishing query tool projects I keep talking about.)

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Speaking as someone who has occupied managerial and/or authority roles, I have to disagree -- at least at "never say no" as an unconditional principle.  

It is perfectly acceptable, even respect-earning, to say "no".  However, you must ''earn'' the right to say "no" by working diligently, consistently, productively and professionally.  

If you consistently do good work, saying "no" -- judiciously, and with justification -- will earn you respect.  It shows you have a clear understanding of your workload, have taken responsibility for your role, are realistic about your capabilities, and are neither obsequious nor excessively agreeable.  It shows you're a professional and not a "yes man". Some weak managers might appear to like "yes men", but ''nobody'' respects them because "yes men" (and women) often bite off more than they can chew and wind up being unproductive.  (That assumes, of course, they were ever productive in the first place...)

Unproductive people -- those who lack diligence, consistency, capability, or professionalism to begin with, or (worse!) who over-work themselves into non-productivity by saying "yes" too much -- do not benefit themselves by saying "no".  From Management's point of view, such an employee is already in a position of having implicitly said "no" too often by failing at his job.  Explicitly saying "no" only magnifies an already negative perception of that employee.

In short: If you're a good employee, saying "no" makes you look better.  If you're a poor employee, saying "no" makes you look worse.

''Well, okay, I agree that you can safely say "no" to those that you have good repoire with and who trust your judgement.''
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Upon close reading of the first post I do not see that he recommends saying "yes". Just not plain "no". The recommendation looks more like "ItDepends". It gives qualified information about the doability. Which is a GoodThing. 
Depending on your position and your need to decide it may be neccessary to say a clear "yes" or "no" such that progress on some issues can be made. Stalling off all decisions will get any manager stuck quickly.

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"Sure, we can make 2 + 2 = 5, Mr. Bernanke, no problem!"

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It is a good rule. Always look for a positive yet appropriate response. That is hard work because you usually have to think on your feet, but it is worth trying. Paraphrasing is the solution of last resort. Of course, rules were made to be broken.

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THis is a CulturalAssumption. There are a lot cultures where saying No is even considered impolite and there are more or less opaque ways to refuse. Clear negative feedback in general has 
the advantage of identifying problems instead of proliferating them
the disadvantage of social instability.

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CategoryEmployment, CategoryPsychology