Google "power of No" and see what happens. You'll see a self-help book, a "negotiating secrets" book, a how-to guide for standing up for yourself, a YouTube clip, and even a domain of the same name...for telemarketing services.
A little help, please?Remember, No is one of the three most powerful (and magical!) words your psyche has; so with magic and telemarketing in mind, let's look at what No is, what No does, and whether No's effects are worthwhile.
What is No?
First of all, only the densest among us needs a freaking book (The Power of No: How to Keep Blowhards and Bozos at Bay) to realize that No is empowering and self-assertive. Anybody who's ever turned down a date or a job offer knows that being desired and being in control only happens with No.
Second, No, by itself, cannot exist. What I mean is, No is always a response. It must react--something or someone must be denied. No is co-dependent and cannot create.
Third, No is an opposing force. Some say (like the book above, and its fans) that No is a key component for not being a doormat; this is sort of true. However, the only reason these people aren't doormats is because they end up being contrarian dickheads--other people avoid them. No is very infrequently a tool of true doormat-prevention. Most often, it is a word of fear or selfishness or laziness. (If this doesn't sit well with you, skip ahead to the next spell in your book: Why?)
Fourth, No is not only a spoken, two-letter word. No is any denial. Close your eyes (not yet, finish reading this first) and picture that pissed off guy muttering to himself and shaking his head; that's No, too.
Fifth, No is a zero-sum game. You can only win if another loses:
I say, "Do you want to grab a drink after work?"
You say, "No."
I lose your company; you gain the comfort of not having to tolerate my imperfect teeth.
What does No do?
No stops motion. It ends conversations. It replaces thoughts and ideas with resentment and argument. In improv, it actually kills the fun...actual, knife-in-the-face, murders the fun. It has an uncanny ability to suck the air out of a room and can make even the most jovial gatherings awkward. During meditation, it bates the breath and the mind's flow. In the LSAT, it lowers the score. In life, it stunts growth.
These are all grand statements, so let me give one example of how No works its magic:
When I was a waiter/actor, I worked in a very fancy restaurant where one plate could cost more than $300. The cheapest drink was a bottle of domestic beer for $9. This restaurant was a different speed than what I was used to.
Anyway, one of the house rules was, "Do not say 'no.'" If a customer wanted anything--anything, we would make it happen. Anything.
This, of course, is insane.
But holy Christmas shit, that nightmare-house of a restaurant was the most creative place on earth. Da Vinci would have wept tears of artist-envy watching us meet some of those demands. (You want a purebred Yorkshire Terrier puppy to pet while you eat your dinner? And you want your desert shaped like your grandmother's silhouette? Absolutely. Never mind that it's 1 a.m., no image of your grandmother is available, and the desert you want is a crème brûlée; the puppy will be here in half an hour and your desert will be ready as soon as you'd like.) Needless to say, the pastry chefs were amazing. But where the fuck do you get a purebred puppy at one in the morning? You make some goddamn phone calls and find out.
Are the effects of No worthwhile?
The answer to this depends on what you want in the time you have left. I put it at 60 years, max.
Back to the example: If we had told the customer "No, there will be no puppy with your dinner," there would have been an argument (I know, it's crazy, but a fight would have happened), and I would have gone home, slumped into the couch, and complained about crazy people and what a shitty workday Wednesday turned out to be.
But because we said Yes, we all walked away with an awesome story and one of the weirder half hours of our lives. Also, instead of going home and bitching, we stayed after work drinking and laughing about it. 
I'm pretty serious. I can't do anything.
If immediate control, visible power, and feeling safe is how you want to run out the clock, then the magic of No is where it's at. (One note: No is temporary; in the end, we all lose control, power, and safety when Death comes a'knockin'.) Though No will sacrifice experiences, growth, and fun, it will more than make up for that loss in self-assertion. People probably won't bother you, but they probably won't seek you, either.
For those of us who want our castle moat filled with sharks, and boiling oil on the turrets, the spell that's most worthwhile is No.