Here’s a mantra to help you re-learn this important word: “It’s okay to say no.” Some people, unfortunately sometimes even our supervisors, seem to think that saying no is an excuse or some kind of cop-out.
It’s funny. We spend the first two or three years of our lives being told no, and then the rest of our days, especially at work, forgetting everything we have learned about that little three-letter word. Saying the word “no” is something you have to learn again if you hope to gain a good work-life balance.
They make us feel like saying no is a personal failing, or even worse a personal insult. Whenever they try that on you, just repeat the mantra: “It’s okay to say no.”
It’s okay to say no when you are overwhelmed and can’t keep up, or when you feel that the request is unfair or unreasonable. When someone asks you to work late on Friday night and they wait until just before 5 to ask, it’s okay to say no.
If you have worked the last three weekends and they ask you to work a fourth, it’s perfectly fine to say no. Saying no is also acceptable if you physically can’t perform a function, or if it involves a task where you do not feel qualified or competent.
Source : http://workawesome.com/work-life-balance-2/work-life-balance-2