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Everyone has been reminded at some point or another, "Don't be a quitter." But for most politicians running for president, there comes a time when, like it or not, you have to be a quitter.

Newt Gingrich is the latest to face this dilemma: beaten, bruised, and battered after a campaign that once looked promising, when to quit? How to quit? Only two months after he surged to a commanding victory in South Carolina, Gingrich only has one other win in his home state of Georgia to make the case for his continued presence in the race. The campaign of Rick Santorum, ahead of Gingrich in the polls and a far greater threat to frontrunner Mitt Romney, has urged Gingrich to drop out, but thus far the former House Speaker refuses to do so.

There's an art to exiting. And it's not always as simple as the moment when you realize you can't win.

Take Gingrich himself, for instance. Even with all his outsized, bombastic confidence, surely he must know by now that there is no way he can beat Romney to the nomination. Gingrich led the race last Thanksgiving, then tumbled to two dismal results at the start of the primary, first in Iowa, then New Hampshire. Gingrich surged to a win in South Carolina, then collapsed in Florida. With 21 states having voted already, it would take a miracle for him to come back at this point - or, in the words of the Romney campaign, an "act of God."

So why won't he bow out? For starters, the key factor in any decision is not whether there's still a chance to win, but whether there's still money in the bank. For now, Gingrich doesn't have that problem - as long as casino billionaire Sheldon Adelson is writing him checks, Gingrich can stay in the race.

The lack of cash flow is frequently what dooms campaigns. Take the very first candidate to drop out of the GOP race: Tim Pawlenty. The former Minnesota governor's third-place finish in the Iowa straw poll last August was devastating, but it was a rising debt that left him no possibility of continuing. He dropped out the morning after the straw poll, saying on "This Week" that his message "didn't get the kind of traction we needed and hoped for." Pawlenty then spent the better part of the next six months traveling the country in support of Romney, who, along with executives at Bain Capital, duly paid off Pawlenty's campaign debt in February.

Pawlenty's aides, however, were left to rue the decision, believing that their boss still had a chance to mount a comeback and win the nomination - after all, when he dropped out in August, it was over four months before the first primary votes were cast. One aide said in December that Pawlenty would be "doing very well" if he had stayed in the race.

At least Pawlenty didn't burn any bridges after he stepped aside. The same cannot be said for Jon Huntsman. The former ambassador to China, who dropped out in January after a third-place finish in New Hampshire, was supposed to speak at a gathering of donors in Palm Beach earlier this month, sponsored by the Republican National Committee, but the RNC retracted the invitation after he told MSNBC that the party was "going to have problems politically until we get some sort of third party movement or some voice out there that can put forth new ideas." Even Huntsman's endorsement of Romney didn't go over too well, as the GOP frontrunner wanted nothing to do with a rival that once called him "completely out of touch."

Another positive about Pawlenty's departure: he didn't waffle, wavering back and forth about what to do. I'm looking at you, Rick Perry. After the Texas governor placed a disappointing fifth in the Iowa caucuses, he announced that he was suspending his campaign to return to Texas to plot his next move. Then, betraying both personal indecision and campaign discord at the same time, Perry tweeted the very next morning that "the next leg of the marathon is the Palmetto State… Here we come South Carolina!" Of course, he dropped out before South Carolina even voted two weeks later, admitting that "there is no viable path forward for me in this 2012 campaign."

We can cut Perry a break, though - it was the first defeat of his 28-year political career, so he didn't have a lot of practice in quitting.

Another issue for politicians to consider in the process is what awaits them once they withdraw. For Perry, it was back to his day job governing Texas. But for Gingrich, for instance, there is no job awaiting him. For another one of this year's GOP candidates, Michele Bachmann, there was a day job, but also a reelection battle.

While Perry went running to make up his mind the day after Iowa voted, Bachmann was cancelling her plans to travel to South Carolina and announcing her intention to drop out. She didn't have much of a choice. Her campaign had called Iowa a "must-win" state, but she placed last in the caucuses, receiving only five percent of the vote and even losing her hometown of Waterloo. At a hastily-arranged press conference on January 4 in Des Moines, the Minnesota congresswoman acknowledged that "the people of Iowa spoke with a clear voice, so I have decided to step aside." Three weeks later Bachmann announced that she would seek a fourth term in Congress.

A whole separate category belongs to Herman Cain. Dogged by claims of sexual harassment and an extramarital affair, the former Godfather's Pizza CEO announced in early December that - even though he'd led in the polls only a month earlier - he was suspending his campaign. "I am not going to be silenced and I am not going away," Cain vowed, announcing that he was shifting to a "plan B." Of course, today Cain's campaign remains "suspended" and his "plan B" apparently consists of making the odd campaign stop in support of Gingrich and little else.

Eric Woolson, who worked on the Pawlenty and Bachmann campaigns last year, as well as Mike Huckabee's bid for the nomination in 2008, said that the confidence and the competitiveness of candidates make dropping out a "very difficult" call.

"Obviously, the decision to withdraw is often a very difficult one, even when a candidate trails badly, because there are a lot of emotions involved," Woolson said. "At the presidential level, you're often dealing with candidates who have never lost a race before. It's hard for them to resign themselves to the fact they've lost. Decisions are often made with the head rather than the head, even when extremely smart people are involved. From a logical perspective, candidates can often still see a path to victory - and that's what keeps them in the race. Certainly, there's a measure of ego involved but, more importantly, you're talking about people who are real fighters. If you're willing to put yourself through the crucible of a grueling national campaign because you feel you're the best person for the job, it is very, very tough to quit."

"So the process often involved a lot of back and forth. How can we stay in? What could we get by staying in? How much is it going to cost if we do - financially and reputation-wise? What are the money people saying? Does it seem like the staff is capable of going on or is its will broken? Is the staff telling me to get out? Often, their family is going to tell the candidate to keep on fighting because they're emotionally wrapped up in it and too close to the candidate. In a sense, it's like the five stages of grief: denial, anger, denial, bargaining, depression, and acceptance."

Sometimes, as was the case with Perry, the aides are left to clean up the mess. In 2000, Steve Forbes left his staff scrambling to cancel over a dozen radio appearances and three television appearances that were scheduled for the day after the multimillionaire decided, in the wake of a third place finish in the Delaware primary, to leave the race.

Ultimately, calling it quits is sometimes easier said than done.

Matthew Jaffe is covering the 2012 campaign for ABC News and Univision.