Whether it's your boss asking you to take on a new project that you simply cannot do, or an employee coming to you with a project that doesn't quite fit the company's needs—how you say no is essential to your success, whether you are a manager or an employee.
Here are some tips on how to say no in the best way possible:
Say what you mean
Don't say yes and when you mean no. If you are reluctant about doing what you have been asked to do, it will show, and you probably won't do it well anyway. Offer another solution. For example, If your boss asks you to work late to finish an urgent report, and you have a date you just can't break, try saying: "I'm sorry I can't work late today, as I have a prior commitment." Then say, "But I can come in early tomorrow and get it done." Be the one to present your boss with a solution to the problem; don't become part of the problem yourself.
Perhaps a colleague asks for help with a presentation, and while you don't want to be uncooperative, you're sick of doing his work for him. Say, "I can't help you this instant, but I've got some time next week when I could show you how to use Power Point yourself." Subtly let the boss know you've made the offer, with the subtext that you are a team player, and not being helpful.
You may also have to say no to someone who reports to you. Use the same process: give the person a reason and offer her a different solution.
Keep your standards
There are occasionally times when you really have to say no because you are being asked to do something that is not merely inconvenient, but involves cutting corners, inappropriate, or unethical. If your boss allows or even encourages such behavior, then it's time to look for another job. Don't wait to be fired for refusing.
If you can be flexible and cooperative most of the time, saying the occasional no will carry more weight. Your boss and your colleagues will know that you are not being difficult and will respect you for it.