In the UK that's illegal. If your boss asks you to work times outside your contract hours, you can take them to an employment tribunal.

@lunchcoma

I, and some others I know, have lost positions (or had our positions threatened) if we say no to our supervisors. Once I was driving to class, and my boss called to ask if I could come in. I reminded her that I had class during the time she wanted me to work, and her response was "So you're going to go to class…

I don't say "No" w/a hard stop. I redirect. "I'm not up for this, but you should talk to so-and-so." Person almost always grateful.

If someone's asking for free professional services, I usually either tell them (truthfully) that I'm not well-informed about that particular area and that they'd be better off discussing it with someone else, or if it is in my field, give them an estimate of how much time it would take for me to work on the project…

Just be honest about it. "I'm satisfied with my level of participation now, but I am not willing to commit to more." No excuses are necessary.

@amolad

Is there going to be a follow-up article on a nice way to back out of something you've already agreed to?

* It is very difficult/complex - Japanese way (also good for politics)

Work in IT and you'll get used to saying No really quickly.