FORTUNE — Dear Annie: I like my job, and I just got a promotion, but I have a dilemma. For the past three years, I’ve been trying to get a foot in the door at my dream company, without success — until now. The company has contacted me with an offer that I really feel I cannot turn down. I know my bosses where I work now won’t like it, but I can’t let this opportunity pass me by. Do you have any advice on how I should break the news that I’m quitting? — In Demand
Dear I.D.: No doubt about it, leaving right after a promotion is awkward. But, unless you have an employment contract that says otherwise, “everybody’s a free agent,” notes Howard Seidel, a partner at Boston-based executive coaching and outplacement firm Essex Partners. “Companies terminate employees all the time, and the flip side is that people are free to quit. You absolutely should not feel bad about doing what’s in your own best interests.”
That said, however, you’d be wise to handle your exit with care. Before you take off, here’s a pre-departure checklist:
Be direct. “The first person to find out you’re leaving should be your immediate boss,” Seidel says. “And have this conversation in person.” When people are nervous about how their news will be received, he observes, “it’s tempting to hide behind technology and call or send a text or an email. Don’t.”
Two possible exceptions, he says: “If you always work remotely anyway and rarely see your boss, or if your boss’s calendar is so packed that you’re afraid you might not get in to see him or her for weeks, then maybe an email is okay — but try to follow it up with an in-person meeting if you possibly can.” Then, when you get there, “watch your tone. Don’t be defensive or apologetic, but don’t be defiant either. Just calmly state the facts.”
“Get right to the point,” advises Jonathan Mazzocchi, a partner in the New York City accounting and finance division of staffing and recruiting firm Winter, Wyman. “It’s like ripping off a Band Aid.”
Mazzocchi recommends writing a brief formal letter of resignation that includes when you expect your last day will be, and bringing two copies to this meeting: “Ask your boss to sign both copies, and you keep one.” Why? “Let’s say you’re in the middle of a long project and your leaving will cost the company money,” Mazzocchi hypothesizes. “Someone at the company may try to make trouble for you by telling reference checkers you were fired. With the signed letter, you have proof you left voluntarily.”
Fend off a counteroffer. Mazzocchi says that, in situations like yours, “companies most often will offer you more money to stay — even though it’s a slippery slope for them, because if word gets out, other people will want the same deal.” From your point of view, he adds, the trouble is that bosses who are already dismayed because you’re leaving will be even more so “if they jump through hoops to make you a counteroffer and you still turn it down. That really does upset people.”
The way to prevent that debacle, he says, is to “be very definite about your intention to leave. Explain briefly what appeals to you about the other job offer, including the fact that it’s your dream job and fell into your lap when you weren’t actively looking, and it comes with certain opportunities that aren’t available in the position you have now.”
You don’t have to say which company you’re going to, Mazzocchi adds: “There are actually people who will call someone they know at the new company and badmouth you, just to be vindictive. It isn’t common, but it happens.” If that seems at all likely, don’t give anyone the chance.
Help hand over the reins to your successor. Even if your new employer wants you to start right away, “don’t give short shrift to this transition process,” Mazzocchi warns. “You may have to work nights and weekends to ensure that you don’t leave your current employer in the lurch, but it’s worth a few hectic weeks of extra effort, because it shows them you care.”
Funny thing about a career: Especially if you stay in the same industry for decades, and sometimes even if you don’t, you tend to run into many of the same people over and over again, and you never know who will be important to your progress later on. So do your best not to trample on anyone’s toes as you make for the exit. “Too many people just run out the door,” says Mazzocchi. “They usually regret it later.”
Preserve your relationships with current coworkers and bosses. Right now, your next employer already wants you, so references may not be an issue. But, notes Howard Seidel, that can change. He has seen executives accept their dream job and then lose it, or decide it’s not so dreamy after all and plan to quit, “in a few months or a year. If that happens, you’ll need good references. So thank everyone you’ve worked with or for, especially if there is someone who has gone above and beyond for you in your current position. Then, after you’re gone, stay in touch. These are essential people to have in your professional network as time goes on.”
Good luck — and, by the way, congratulations!
Talkback: Have you ever had a bad experience quitting a job, or been left hanging when someone else quit? Leave a comment below.