FORTUNE — Dear Annie: I hope you don’t think this is a dumb problem, but I need some advice on how to refuse when people ask me to do things that aren’t part of my job description, without being rude about it or burning any bridges. Over the past several years, I’ve gradually become the “go-to” person in my department for more and more extra tasks. I’m happy to help out, even though it usually means staying an extra hour or two in the evenings to get everything done.
But now, my family has my elderly and ill father living with us, and I need to leave work at 6 p.m. on the dot to relieve the home-care nurse. So I want to bring up the idea of dividing the extra work I’ve been doing among the five other people on my team — which sounds simple, except that all my life I’ve had trouble saying “no” to anything that anyone wants me to do. It’s always been easier for me to just say yes than to deal with the feeling that I’m letting someone down. Do you or your readers have any advice? — Doormat in Denver
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Dear D.D.: First of all, I don’t think this is a “dumb problem,” and neither does Bob Burg, a speaker and author who frequently hears some variation of this question from audience members in his seminars at Fortune 500 companies. “Women tell me they have an especially hard time saying ‘no,’ but I think it’s a problem for men, too,” Burg says. “They just don’t admit it as much.
“Most people are generally nice and don’t like to disappoint other people,” he adds. “Where it gets counterproductive is when that impulse takes over and makes us do things that aren’t in our own best interests.”
Moreover, the situation you’re in — where a job just keeps expanding, little by little, until you’re loaded down with all kinds of extra duties that aren’t technically your responsibility — is so common that it actually has a name: Scope creep. “You often hear people in client businesses complain about this,” says Burg. “It happens a lot in consulting, where there are certain set expectations at the outset of a project, but the scope just keeps expanding until it’s gone way over the line of what was agreed upon or what’s reasonable.”
So how do you cure your scope creep? Burg, who wrote a book called Adversaries into Allies: Win People Over Without Coercion or Manipulation, takes issue with the notion, popularized lately by Oprah Winfrey and others, that “No is a complete answer.” “I cringe when I hear that. Saying ‘no,’ period, when someone asks you to do something is rude. It also tends to alienate people, which can make your job more difficult and cut you off from future opportunities.”
A better approach, he says, is to call an informal, five-minute meeting with your five coworkers and explain that, from now on, you’ll be leaving the office no later than 6. “Frame this announcement as an ‘I’ message,” Burg suggests. Prepare a little speech beforehand where you say something like, “I need to bring something up. I’ve allowed myself to take on a lot of extra tasks here and, while I’ve enjoyed helping out, it’s an issue now because I have to leave at 6 for family reasons.”
“Notice how often you say ‘I’, rather than ‘you,’ as in, ‘You all have been dumping too much extra stuff on me for a while now,’” says Burg. “Focusing on the ‘I’ keeps people from feeling they’re being accused, which will just make them defensive” — and less likely to go along with your plan to redistribute the workload. Then say you’d like to work out a system for sharing the extra work equally among the group, and ask for suggestions about how best to do that.
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Particularly since you’ve always found it difficult to say no, “the toughest part is going to be the first couple of times after the meeting when someone tries to hand you one of those extra tasks,” Burg says. “But be steadfast. Remind the person politely that you’ve already said you’re no longer the only one responsible for whatever it is. Don’t apologize, and don’t give in.
“What gets rewarded, gets repeated. We train people how to treat us, so if you always say yes, you’re training these folks to keep asking,” he adds. “But once you’ve retrained them, by gently reminding them that the situation has changed, you should have no problem.” Here’s hoping.
Talkback: Have you ever experienced “scope creep” at work? What did you do about it? Leave a comment below.