Some management topics never go out of style. And like fine wines, they only seem to get better, and more potent, with age. For instance, what is it going to take to get Hillary Clinton to finally figure out that she isn’t going to be the Democratic nominee for president?

Maureen Dowd of The New York Times raised the issue again this week. “Whoever said that after denial comes acceptance hadn’t met the Clintons,” she wrote. “If Hillary could not have an acceptance speech, she wasn’t going to have acceptance.”

In the business world, Hillary Clinton’s behavior would be par for the course. CEOs and other high-ranking executives seem unable to leave, despite overwhelming evidence that they have lost their effectiveness and ability to lead. It was true of Bob Nardelli when he was at Home Depot, Paul Wolfowitz at the World Bank and even John Mackey of Whole Foods, a guy who continues to hang on and mouth unrepentant gibberish defending his indefensible behavior.

In fact, it’s amazing when someone in politics (or business) actually decides to do the right thing and get out sooner rather than later, as I noted in January in writing about John Edwards’ magnanimous decision to end his campaign for president.

What I said then about Edwards is even more true about Hillary Clinton now: As with all things in life, there is a time to dig in and fight, and a time to let things go. No one wants to be thought of as a quitter, but then again, no one wants to stick it out and continue to battle beyond reason. The trick is knowing when there is still a good reason to fight, and when to resign yourself to the inevitable and let things go.

There’s a lot Hillary (and Bill) could learn from John Edwards about making a graceful and genuine exit, but I’m afraid that she has already missed that lesson—especially the part about putting your own personal ambitions aside for the greater good of the organization.

The latest word is that the New York senator will finally concede to Barack Obama sometime this weekend. Maybe that’s true, but recent history is riddled with examples of leaders—corporate and political—who should have known it was time to depart gracefully, as John Edwards did, but failed to do so. Hillary Clinton is just the most recent case. Sadly, she will not be the last.