I am aware that there is pressure in this culture to let somebody down easily. I think this is a dangerous practice. A refusal is not a particularly fine art and thus requires very little practical advice. However, if you find making refusals difficult, or find yourself against a foe who does not seem to grasp the concept that you do not wish to, consider using the 3, 2, 1 structure.
A first refusal rarely requires more than three words, “no, thank you,” should be sufficient. Other phraseologies may be used depending on the tone of the request you are refusing. “I’ll pass, thanks,” “no, you twit,” and “no, fuck off,” are all acceptable under different circumstances.
In many cultures, it is polite to follow a first refusal with a request for confirmation, frequently along the lines of “are you sure?” In these cases, the two words that are best employed are “I am.” If the speaker instead seemingly has ignored your initial refusal, you might choose to use stronger words than “no, thanks.” “Go away,” and, more simply, “fuck off,” usually work.
If a three-step refusal is required, never spend more than a single word on the last stage. “No,” is the only word you should need, anything else is passementerie. Needing anything more than the repetition of “no” in the final stage of a refusal is an indication that passersby ought to interfere as it is likely that anybody who does not accept the three-stage refusal is suffering some sort of derangement.
As a final note: NO is not a bad word. NO is a powerful word. Anybody who discourages you from saying NO does not have your best interests at heart. Say it loudly, say it often, and say it proudly.