Saying “no” is one of the best antidotes to the “I’m so busy” problem we have in this country and around the world. 

You should say no to a coworker when:

They ask you to help them cover their mistake. You say no because it’s a cover-up and lacks integrity. While it can be embarrassing to acknowledge a mistake or have it called out, it’s better to unwind the mistake than to hope that it won’t come back to bite you. “I can’t help with that and you should give it more thought before you decide what to do – getting caught not owning the error could be much worse”.  

They suggest you fudge a number or move a sale into a different period than it actually occurred. No. This is an integrity issue. If it’s your sale – congrats – but only in the period you earn it. “I’m not going to do that. It would have been nice to hit the incentive this month, but I guess I”m off to a head start for the next tracking period”.

They want you to share private or confidential information (expressly confidential or assumed to be confidential). Gossiping is universally bad and the best way to say no is directly – “I’m sorry, I’m not familiar with that” or “That’s not something I feel comfortable sharing”.

They want you to participate in any social activity (drinking, drugs, teasing, absenteeism, etc). Social events, fun and happy hours are fine when they are professional and handled with moderation. That said, there’s always a party crowd or social bunch that sticks together. You have to use your judgment and keep your professional and personal reputation in mind. Saying no if you have other work to do or if you simply don’t want to participate is easy here. “Oh I wish I could but I have another commitment” is the safest and most politically correct. You don’t have to be the fun police, but you also don’t have to jump off the bridge when a friend tells you to.
Say it politely and avoid tones of judgment or disapproval. If it’s your boss and it’s a request for you to do something you need to say “no” to, explain your reasoning and also try to provide alternative solutions or options to deflect the request away from you. If it’s an appropriate request, perhaps there’s another team member who could handle this task. 

Don’t be the party of no. Some people say no to avoid work or avoid change. In sales, service, and team-based work, it’s important to get to yes to keep projects and business moving forward. It’s best to find other ways to cooperate or say yes when working with clients and colleagues but nobody likes or expects a “yes man” or “yes woman”. However, you do need the wisdom to know when and how to say “no” with grace.