(FORTUNE Magazine) – In the world of CEO succession planning, companies generally need to contend with two possibilities. The first, and more common, is the relatively predictable retirement of a chief executive. More troubling, however, is what's known as the "hit by a truck scenario." That's when the board of directors has to act fast to right a company in a leadership crisis. Either way, proper planning can make all the difference. Researchers at Spencer Stuart, the executive search firm, recently completed a study of ten companies that plan for succession particularly well. Here's what they learned:

--Sic transit gloria. Like the rest of us, CEOs are generally reluctant to face their own mortality. But they should probably think of succession this way: things will be really bad for the poor company when they go, so it's their responsibility to make sure they have a smart strategy for the transition. Money may help them get the point; Spencer Stuart has found that boards are tying as much as one-third of the CEO's bonus to the development of a thorough succession plan.

--Levels of talent. Bench strength is important, so it pays to keep track of who's coming up in the ranks behind the top 15 or 20 people. "Our top 100 people usually have at least two or three possible successors each," notes J. Randall MacDonald, GTE's executive vice president for human resources and administration. "So our CEO spends a significant amount of time reviewing the progress of at least 250 people."

To help him in that task, MacDonald and others at the company maintain huge notebooks filled with all manner of data on the top contenders. "There's everything you'd ever want to know in there, plus more," he says. "We've got significant accomplishments detailed, critical skills, assignments they need for further development, and some guesses as to which position we think they'll top out in."

--Look to the outside. Just because a company has a strong bench from which to promote doesn't mean up-and-comers shouldn't be benchmarked against their peers at other companies. "It's reassuring to know there isn't a candidate someplace else who has something wonderful that our people don't have," says Catherine Rein, an executive vice president at Met Life.

--Let the board play. Since the directors will ultimately pick the next CEO, it's crucial to regularly parade various contenders in front of them. "Our senior officers are constantly making presentations to the board," says Mellon Bank CEO Frank Cahouet. "The board sees the executives' results against their plans or dreams and learns how they conduct themselves when they're faced with difficulties." It's also smart to encourage front-runners to serve on another company's board. After all, their first experience working closely with directors should not be on the day they're promoted to chairman.

--Keep it quiet. While Cahouet passes a sealed envelope to the board each year, detailing his choice for a successor, he and others don't believe in publicly naming the next in line. "People change and situations change," he says. "If you've made your choice and then are reluctant to admit you've made a mistake, you can't fix it without creating considerable embarrassment." If you need proof of that statement, just consider one name: Michael Ovitz.

--Ronald B. Lieber