Just say it... well, that was easy. Let me explain.
From the time we can understand language, we hear the word no more than any other word, and as we grow a little more into our childhood we receive repercussions for saying it. Think about it - did you ever tell your parents no and their response was a hug and a smile? This behavior is why we are conditioned to think we can't challenge authority figures.
In general, our society inherently places a negative stigma around declining just about anything... from an invitation, to help with a project or a simple chore around the house. The pressure intensifies when you are objecting authority in the workplace. To be clear, I am all about being a team player, so there are definitely times when you may be asked to do a little extra here and there and if you can, I recommend stepping up and helping out. However, if you are asked to do something unreasonable, you are allowed to decline. Just because the request comes from your manager doesn't mean you're required to comply.
Although you can say no, there is a right and wrong way to do it. For example, instead of flat out saying "nope", you could respond with "Unfortunately I'm unable to complete [insert task] at this time". In my experience, I find that communication is half the battle - when we communicate effectively we are able to resolve more issues, faster. Check out my Communication Quick Reference Guide below for more examples.
As a HR professional, it saddens me to hear the bs people will put up with out of fear of losing their jobs or facing childish backlash for speaking out. If you are ever in a situation when you feel harassed or pressured to perform outside of your job description excessively, you have the right to report your aggressor to your human resources department. When colleagues approach you for help, it's always your choice to help - no one, not even your manager - can force you to assist on a project or additional task that is not within the scope of your role responsibilities.
However, as an employee, I understand the negative stigma around speaking up against the pressure. I've served on many teams that ran lean (meaning we were understaffed and overworked), so if we didn't constantly lend a hand here, and stay late or work through vacations we just couldn't keep up with what needed to be done. Could I run to my HR - well we are HR so not exactly - but in theory yes, I could have found someone in upper management to confide in about these issues. I chose not to do so because: 1) I wasn't the only one feeling the heat of the kitchen; and 2) I look at extra responsibility as an opportunity to grow.
The last important factor in protecting your limits is courage. Although you may understand you have the right to refuse additional work without consequence, and you want to protect your peace, you still have to muster up the confidence to speak up. This is honestly the biggest reason why most people don't tell their boss no; they would rather feel overwhelmed and flustered than look their manager in the eye and respectfully deny their proposition. Sometimes it's because we feel saying no makes us look selfish. Other times we fear saying no could hurt the personal relationship and weaken the trust between you and your manager. Whatever the reason, it takes strength for your thoughts to form into articulate words, and for those phrases to then come out of your mouth (or be written in email).
Understand this - your energy and peace of mind are the most valuable assets you have. Saying no will also set your personal boundaries, showing your colleagues where your limits are. As long as you are a good employee, there is no need to overload yourself trying to be the office "Yes" (wo)man. We get so caught up in our routines we forget we were hired to do this job because the company saw us as the best candidate. You're already a winner... don't forget that. Come to work and do your job well. If you're a team player and you put out positive energy you're going to be respected. Remembering this will save you a lot of unnecessary stress and headache.