(FORTUNE Magazine) – That dream job you've been hoping for is finally yours. Congratulations--maybe. Well, now, how--and what--do you tell your boss, and how do you cut the cord without severing a relationship? That's important--no one wants to tarnish a hard-won reputation as an ethical, classy person.

Before you jump, be confident that you're making the right move. Probe a prospective employer as rigorously as he or she queries you. The most important part in evaluating a job offer, advises headhunter Gary Knisely of the Accord Group, is to learn all you can about the company wooing you and the specific job it wants you for. He figures that only one out of ten candidates does that task well. Echoes outplacement expert Robert Swain of Swain & Swain: "People suspend critical judgment. They're not very rigorous about the pluses and minuses of the companies that are romancing them."

Check out what the company says about itself on its World Wide Web site. Then ask your prospective boss for references. After all, she should recognize that interviewing is a two-way street. Speak with suppliers, customers, and former employees. Security analysts can be penetrating judges of the company's prospects. Says Knisely: "Analysts are always talking with senior officers, and they're pretty unbiased." Get an organization chart and ask your prospective boss and potential peers--yes, speak with as many of them as possible--exactly where you will fit in. Inquire of your boss what you can reasonably expect to grow into in a couple of years.

Don't take a new job just for more money, unless the step-up in pay is huge--at least 20%. And leave a good job only if you're sure to gain more authority, which means being close to your new employer's real power source. For example, says Gemini managing director Harry Moser, "in management consulting, the client is king. If you're being recruited for an administrative position, try to reshape the job so that you won't get detached from the clients."

When you decide to accept the new job, tell your boss--and don't try to lever it into a negotiating session for higher pay. Sure, you may get a counteroffer, but if you accept it, your boss may later resent you for extorting it from him and consider you disloyal. "In too many cases," says Swain, "it's never forgiven."

It's best for all if you simply say, "I have decided to accept a very attractive offer from Acme Electronics. I've thoroughly enjoyed my employment here. What can I do to make this an orderly transition?"

What bosses do resent is being ditched with too little time for transition. The higher your rank and the more people reporting to you, the longer the notice you should give. Middle managers might depart after two weeks, but higher-ups should offer to stay at least two months. You'll need that time to train a successor or help to recruit one.

Co-workers will expect you to take your foot off the pedal the moment you quit. But you want them to remember you for running at full speed, dedicated and caring till the end. Try to finish all ongoing projects; for example, complete your performance reviews and leave written records. Going out with class will make your former boss think even better of you--and you never know when you might need her again.

Of course, if you're jumping to a direct competitor, the scenario changes: It's hand over your keys and credit cards right now, clean out your desk within an hour, and Dick Tracy will escort you to the door. One tip: In such cases, take home your Rolodex beforehand; some companies consider it their property. Law firms and advertising agencies are especially brutal with defectors because employees often take clients with them.

And what if you're one of those poor souls, rare but increasing in number, who take the jump but then decide they have made a mistake and yearn to leap back to their previous employer? "Good luck!" snorts Knisely. "Adultery has been committed. Besides, somebody is probably already doing your job."

Once you join your new employer, act as if you've never been married before. Avoid saying, "as we used to do at my old company." Don't tell tales about former colleagues. If pressed by higher-ups to share confidential information, speak in harmless generalities. Commandments I, II, and III are: Thou shalt not raid thy old company--unless a former colleague approaches you first. Never burn bridges, if you can possibly avoid it. Keep up good relations with your ex-employer. Phone your close co-workers every several months (unless they are now competitors), perhaps helping them recruit talent from the outside. Particularly in these days of frenzied M&A, you may be thrown together again. After all, it's a long career and a small community.