Latest CEO accessory: A chief of staff

(Fortune Magazine) -- In headier days a CEO's must-have accessory was a Gulfstream V or a house on Nantucket.

These days it's a chief of staff, a top-level adviser who's part confidant, part gatekeeper, and part all-around strategic consultant. While that has long been a key position in politics, many top executives are now adding this person to the payroll.

Goldman Sachs (GS, Fortune 500) has a chief of staff. So does Aflac president and COO Paul Amos II. When Tim Armstrong took over AOL in the spring of 2009, one of his first hires was his chief of staff. And Tom McInerney, the new COO of ING Group's global insurance business, has a new chief of staff in the Netherlands at ING headquarters.

Besides being what he calls a "general mopper-upper," Gilt Groupe's Rob Deeming, whose title is director of strategic projects but who acts like a chief of staff, helps CEO Susan Lyne with projects such as coming up with new product lines to sell on the web.

At AOL (AOL), chief of staff Maureen Sullivan, who joined in March 2009 -- and who held the same post for Armstrong when they were both at Google -- leads strategic planning.

Joost Heideman, chief of staff for ING's (ING) McInerney, serves as a confidential "sounding board" for his boss, who recently relocated to the Netherlands from the U.S.

As the first chief of staff at Aflac (AFL, Fortune 500), Angela Kates acts as the link between Amos and the insurance company's 64,300 agents and employees, dealing with issues brought up in the frequent e-mails that Amos invites them to send.

But she also serves as her boss's alter ego, with the power to decide which day-to-day decisions she can make on her own and which require Amos's signoff.

The hardest part is learning how to think like a top executive -- and how to handle as many responsibilities as she can without overstepping her authority. "That is the art of that job," says Amos.

While the chief of staff is a boost to the boss, it's also a two-way street for his helpers. "I get exposed every day to the way a CEO operates," says Deeming.

That mutual benefit means more chiefs of staff could end up in the executive suite one day soon.

"When I first started talking to my HR department, they said that with the exception of politics nobody really [has one]," says Amos. "I said, 'I promise you this is needed out there.'"